"translate" osm server errors into human readable strings

[ulfl]

Currently we have a lot of server problems, resulting in errors if you try to download osm data.

While a common josm user might know what a HTTP 404 message means (however, it's still hard to predict what to do then), a "Server returned HTTP response code: 500 for URL ..." gives you no idea about the problem and what to do now.

So it would be a good idea to "translate" these codes into something more human readable:

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Internal Server Error - please try again later

Server returned HTTP response code: 500 for URL ...

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