[0.2.1-deb] Crasher: Download after deletion

[eloquence]

Encountered during QA.

1. A deletion job failed on the first try.
2. It eventually succeeded, but hit an AppArmor issue ([0.2.1-deb] AppArmor denial causing deletion to not take effect #856).
3. A subsequent download attempt of a file on a different source caused a crash , see logs. (For the avoidance of confusion: the test file was in fact an .so file.)

[redshiftzero]

hmm this seems related to #717

[redshiftzero]

Tried a couple things:

• I thought this might be very similar to Deletion in client and re-submission from same source results in error #717, so I deleted a source while a long-running file download was in progress. This worked as expected, the source is deleted, the file download fails but the user is notified gracefully, the job is removed from the queue and no crash occurs.
• Next I thought perhaps a FileWidget from the deleted source was not getting deleted for some reason, and so the on_file_download slot was running, referencing a File object that no longer exists and causing the crasher (this does seem from the traceback to be what is happening). Tried this scenario in both my dev env and using the python3-pyqt5 package (in case there was divergence from pyqt5), but all works as expected. No errors or crashes.

Regardless of this specific bug it is problematic that an exception occurring in a slot will apparently crash the entire app, we should fix that to log any exceptions.