initial attempt.

// @sindresorhus @CWSpear 
Thanks for catching that! I forgot to point that out and ask how to handle that.

I believe that should be categorized as an error; it's something that makes the task fail, so we should handle it appropriately. I believe just `config.get('emitter').emit('error')` - this will allow devs to catch this event, should they want to print out a pretty error / ignore the error. In `grunt-wiredep`, we can do just that, and print out the "The file was not injected" message. Same for the `wiredep.stream` piece, as well, I believe.