[TW-1473] More detailed on-modify hook error message.

_Eric Nielsen on 2014-12-17T10:17:26Z says:_

I wanted to report a rather cryptic error message when creating a on-modify hook. I was just testing out the hook function and was printing to the console, but was not returning JSON data and I got this error message:

"Taskwarrior no longer supports file format 1, originally used between 27 November 2006 and 31 December 2007."

Once I started returning JSON then it worked fine. But maybe a more descriptive error message like "Modification hooks need to return JSON" or something similar would be better?