thor has a zero return code when task isn't found

[sconover]

(I hope this is the right place to submit issues. If it's not I'd be happy to resubmit in the appropriate place)

$ thor task doesnt exist
The thor:runner namespace doesn't have a `task' task
$ echo $?
0

This is an unsuccessful case, and the return code ought to be nonzero.

[wycats]

Good call. Let's fix this ASAP.

[josevalim]

Fixed!