automatically load tms tile from lower zoom level if current zoom level doesn't provide tiles

[Klumbumbus]

see also ​https://github.com/osmlab/editor-layer-index/pull/290#issuecomment-288808627

What steps will reproduce the problem?

1. have a tms source without explizit max_zoom set
2. zoom in far
3. add the source

What is the expected result?

tile from highest zoom level

What happens instead?

403 error is displayed

Please provide any additional information below. Attach a screenshot if possible.

only after you zoomed out to the highest zoom level of the source the tiles are loaded. after this no 403 errors are displayed anymore

The reason to not explicitly set the max_zoom is that we maybe don't notice when the imagery provider increases the maxzoom and then we don't present the best tiles to the user.

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2017-03-23 00:01:04 +0100 (Thu, 23 Mar 2017)
Build-Date:2017-03-23 02:32:15
Revision:11772
Relative:URL: ^/trunk

Identification: JOSM/1.5 (11772 de) Windows 10 64-Bit
Memory Usage: 1832 MB / 3641 MB (833 MB allocated, but free)
Java version: 1.8.0_121-b13, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM
Screen: \Display0 1680x1050
Maximum Screen Size: 1680x1050
VM arguments: [-Djava.security.manager, -Djava.security.policy=file:<java.home>\lib\security\javaws.policy, -DtrustProxy=true, -Djnlpx.home=<java.home>\bin, -Djnlpx.origFilenameArg=C:\Program Files (x86)\josm-latest-mehr-RAM.jnlp, -Djnlpx.remove=false, -Djava.util.Arrays.useLegacyMergeSort=true, -Djnlpx.heapsize=1024m,4096m, -Djnlpx.splashport=50386, -Djnlpx.jvm=<java.home>\bin\javaw.exe]
Dataset consistency test: No problems found

Plugins:
+ AddrInterpolation (33160)
+ DirectDownload (33160)
+ DirectUpload (33182)
+ FastDraw (33182)
+ HouseNumberTaggingTool (33160)
+ Mapillary (v1.4.2)
+ OpeningHoursEditor (33185)
+ Tracer2 (33004)
+ alignways (33182)
+ apache-commons (32994)
+ apache-http (32699)
+ buildings_tools (33004)
+ contourmerge (1030)
+ editgpx (33004)
+ imagery-xml-bounds (33004)
+ imagery_offset_db (33004)
+ jogl (1.0.46)
+ log4j (32699)
+ measurement (33088)
+ osm-obj-info (1484152384)
+ photo_geotagging (33088)
+ photoadjust (33088)
+ reltoolbox (33088)
+ reverter (33088)
+ tag2link (33035)
+ tageditor (33021)
+ tagging-preset-tester (33004)
+ terracer (33088)
+ turnlanes-tagging (254)
+ turnrestrictions (33088)
+ undelete (32699)
+ utilsplugin2 (33182)
+ wikipedia (33098)

Tagging presets:
+ https://josm.openstreetmap.de/josmfile?page=Presets/OneClick&zip=1
+ https://josm.openstreetmap.de/josmfile?page=Presets/StolpersteineLight&zip=1
+ https://josm.openstreetmap.de/josmfile?page=Presets/NewTags&zip=1
+ %UserProfile%\Documents\OSM\JOSMSVN\data\defaultpresets.xml
+ https://raw.githubusercontent.com/yopaseopor/traffic_signs_preset_JOSM/master/DE.zip
+ https://josm.openstreetmap.de/josmfile?page=Presets/Industrial&zip=1
+ http://somewhere.dk/email/josm/source.xml

Map paint styles:
+ %UserProfile%\Documents\OSM\JOSMSVN\styles\standard\elemstyles.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/HiDPISupport&zip=1
+ https://josm.openstreetmap.de/josmfile?page=Styles/Coloured_Streets&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Maxspeed&zip=1
- %UserProfile%\Documents\OSM\eigene styles\maxspeed\maxspeed_2.8_01 zeige richtungspfeile.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/Lane_and_Road_Attributes&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/sac_scale&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/ShowID&zip=1
- %UserProfile%\Documents\OSM\eigene styles\PriorityRoad\PriorityRoad_1.0.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/LayerChecker&zip=1
- %UserProfile%\Documents\OSM\eigene styles\layer checker\layer checker_1.2.mapcss
- %UserProfile%\Documents\OSM\TestNew\newicons.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/Surface&style&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/ParkingLanes&style&zip=1
- %UserProfile%\Documents\OSM\patches\old MPs\dataquality.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/Sidewalks&style&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Cycleways&style&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Osmc&style&zip=1
- https://raw.githubusercontent.com/species/josm-preset-wheelchair/master/sidewalks_kerbs.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/LitObjects&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Lit&style&zip=1
- %UserProfile%\Documents\OSM\eigene styles\Tourenplanung.mapcss
- %UserProfile%\Documents\OSM\eigene styles\SpecificBuildingValues\SpecificBuildingValues.mapcss
- https://josm.openstreetmap.de/josmfile?page=Styles/Coloured_buildings&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Modified&zip=1
- https://github.com/bastik/mapcss-tools/raw/osm/mapnik2mapcss/osm-results/mapnik.zip
- %UserProfile%\Documents\OSM\eigene styles\area-symbol.zip
- http://www.freietonne.de/ft_icons/josm/FreieTonne_rules_presets_zip.php
- https://josm.openstreetmap.de/josmfile?page=Styles/NewHighwayColors&zip=1
- http://www.openrailwaymap.org/styles/standard.zip
- https://josm.openstreetmap.de/josmfile?page=Styles/MaxspeedIcons&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/DestinationSignRelation&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/ParkingLanes&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Incline&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/PTStops&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/AdvertisingStyle&zip=1
- https://www.dropbox.com/s/qo3ai47fpv241jf/Styles_Fixme_and_Notes.zip?raw=1
- https://github.com/gmgeo/osmic-josm-style/archive/master.zip
- https://josm.openstreetmap.de/josmfile?page=Styles/hazmat&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Coloured_Suburb&zip=1
- https://josm.openstreetmap.de/josmfile?page=Styles/Coloured_Postcode&zip=1

Validator rules:
+ https://raw.githubusercontent.com/<user.name>n-a-bauer/josm-validators/master/mtb.validator.mapcss
+ %UserProfile%\Documents\OSM\TestNew\new.validator.mapcss
+ https://josm.openstreetmap.de/josmfile?page=Rules/GermanySpecific&zip=1
- %UserProfile%\Documents\OSM\TestNew\germ.validator.mapcss
+ https://josm.openstreetmap.de/josmfile?page=Rules/CzechRepublicAddressSystem&zip=1

Last errors/warnings:
- W: java.net.MalformedURLException: unknown protocol: c