Skip files which are not available anymore

Depending on the license set by course instructors, files are made unavailable 13 months after the course start. Currently the html error page is saved when trying to download such a file.

Expected behavior: Skip these files.

A quick investigation revealed that actually the correct response code 451 (Unavailable for legal reasons) is returned. I suspect one could simply check for a 200 respose but just filtering 451 would solve this specific error.