Appends exceptions to a list instead of overwriting the previous value
Saves worker name as well as the traceback for exceptions

I have decided to keep old exceptions as simple strings, but it should not be
common to have this in a real database, since documents are rarely reanalyzed.

This and the previous commit should close #162
Merge pull request #163 from flavioamieiro/fix/exception_logging

Append (instead of overwrite) exception logging

Fix #162