Handling of invalid archive files should be improved

[cg2v]

If a student submits a file that is considered an archive, but the archive is invalid, then submissions#view throws an exception (and displays donkey kong)

In addition to catching and displaying an appropriate error for this exception, it may make sense for validateHandin to reject archives that trigger exceptions from Archive.get_file_hierarchy. (that's not perfect - a zip file could have a valid directory but still fail a later extract, but it's better than what we have now)