WMS: Improve exception handling

[olek.lorenc]

When the server sends an exception, rather than just sending the error to the console (where it can get lost as JOSM keeps requesting more tiles) and displaying the red "error tiles": parse the exception (format: application/vnd.ogc.se_xml) and display more useful information in the "error tile", i.e: no version parameter, no such layer, etc.

[anonymous]

Could bung error information onto the red tile graphic..

Alternatively maybe a little clickable error icon could appear on the layer in the layer panel, taking you to a window full of details.

This, along with bug #3915, would be a good improvement to use-ability for people ​like this struggling with WMS URL problems.

[Don-vip]

In 7425/josm:

fix #3916 - WMS: Improve exception handling. Proper message is now displayed on tiles.

Screenshot:

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