Raise an exception when taskwarrior has a non-zero return status.

Feel free to reject this pull request if this doesn't line-up with your model for how things should work -- I promise I won't be offended.

When working on inthe.am, I needed to be aware of situations when taskwarrior had exited with an error code, so I [subclassed your TaskWarriorShellout](https://github.com/latestrevision/inthe.am/blob/master/inthe_am/taskmanager/taskwarrior_client.py#L66) in my application to add this feature.  Initially, I was going to leave this in my subclass rather than trying to push this change upstream.

Upon further consideration, I decided to send you this PR.  It seems likely that, rather than inspecting the output of the command to see if it was successful, this might provide a more direct way of becoming aware that one's task did not execute successfully.

Cheers,
Adam