2358: Don't crash when uploading translations from non-public languages

Changed it so that the form raises a ValidationError in this case,
instead of crashing when self.from_sv is None.

Changed it so the only valid choices for source translations are
public languages.  This matches the choices when you start the task in
the editor, so I think it's correct.
Fix for query to pull languages.

The previous one would have duplicates if we had multiple subtitle
versions.
Don't crash if a task has been deleted in TaskUploadForm
Merge pull request #166 from pculture/x-2358

X 2358