Catch errors when committing the database
Fix a few missing 'from' clauses in raises
Print exception tracebacks in debug mode
Move .latexrun.db into the output directory (e.g., latex.out)

This way there's only one file created in the source tree.  The only
potential downside of this is that the user can change the output
directory on the command line, which will also change which control
database latexrun uses, but the consequences of this seem minimal.

Closes #11