Reinterpret status code 500 as missing data to avoid long-running retry loops.

The harvester got stuck doing retries tonight as fully empty maps trigger a 500 internal server error at the host which we then dutifully retry, c.f. the examples of https://www.floraweb.de/php/download_tkq.php?suchnr=684 and https://www.floraweb.de/php/download_tkq.php?suchnr=34357.

This change special cases the 500 response for fetching bounding boxes to avoid the undesired behaviour.