Add fail notification hooks before tasks run

This will resolve #39 where running a ```grunt-contrib-watch``` task with ```spawn:false``` would not produce notifications on failure.

I'm not entirely sure if this is the best solution, but it's what I could come up with in a limited amount of time.

I see that there are sample watch tasks in the Grunt configuration file, but I didn't see that they where used in any tests. Perhaps someone can help set these up with some tests. Will see if I can get to it myself when I have time.