Added ability for MPC writer to write lines when photometry has failed
where the observed magnitude and band are left blank.
Photometry failure error handling.
If daophot cannot compute the magnitude when phot_mag is called
a TaskError is raised.  The controller catches this and launches the 
accept source dialog with the observed magnitude and band left blank,
and the comment pre-filled with the TaskError error message.
Merge pull request #97 from drusk/phot_failure

Closes #78.