Handle WMS requests with HTTP error more efficiently in timgen

[jirik]

Expected behavior
If WMS request in timgen returns HTTP status code 4xx or 5xx, timgen retries to send the request and if it fails, it stops immediately (by writing error message to canvas_data_url_error).

Found behavior
If WMS request in timgen returns HTTP status code 4xx or 5xx, timgen does not retry the request and it does not write error message to canvas_data_url_error. Therefore thumbnail.py waits for all tries and this waiting is unwanted.