Calling that add_files method of Submission model causes DatabaseError in case the file_object of SubmittedFile is too long for the database. I am not aware of other kinds of DatabaseErrors that add_files may catch but I wouldn't be surprised if those exist. Thus the error message has uncertainity in it.
Added exception logging for some caught exceptions and edited some user error messages. I am not sure if the the level of the logging is appropriate but it is at least an improvement.
Removed done todo-item.
Merge pull request #78 from Rubinous/some_error_handling

Some error handling