Improve user feedback in case of tile server access failure

[namdre]

Currently, sumo-gui may generate errors of the form jpeg' not recognised as a supported file format or it may simply fail to show the background.
Related: #6481, #6279

Ideally, sumo-gui would display a warning to the effect:
Warning: Background images could not be loaded (You don't have permission to access arcgisonline.com)

Where the explanation of the warning is the error text in the failed 'jpeg' download.

(Maybe such an error message would wrongly suggest that the images are downloaded from within sumo-gui which also isn't ideal).