Enhance error message on parse failure

[satyagraha]

Very nice project, thanks for the effort.

Would it be feasible to include the column name, or column number if not present, in the error message on parse failure? This would help a lot in tracking down issues. Thanks.

[pjfanning]

@satyagraha could you make the fields in your case class optional? Then you'll be able to check for yourself which values are None.