Improve handling of "The area you tried to download is too big or your request was too large" error

[Geimas5]

Currently, when you try to download data for an area which are too large, the user gets a message like this:

"The OSM server 'api.openstreetmap.org' reported a bad request. The area you tried to download is too big or your request was too large. Either request a smaller area or use an export file provided by the OSM community."

Afterwards, the download dialog is closed, and the user has to reopen the dialog and try again.

An improved scenario would be that the download dialog remained open, with the area still selected. This would make it easier for the user to download a slightly smaller area. It is very unlikely the user does not wish to download any data at all after receiving this message.

This would also help the user select a better area size the next time. Indeed, I often get the message several times in a row because I might not remember exactly which are I tried to download.