fixes #167; tarbell generate throws a failing exit status if templates don't render

Hey! I can understand if no one wants to merge this pull, as I don't know your guys' workflow or anything. I really wanted to have this functionality, and to me it seems intuitively like the right way for `tarbell generate` to behave. 

With these changes, Tarbell fails fast during a `generate` or `publish` command if a template isn't able to be rendered. I tried to alter as little as possible to get it to work. Feel free to rework these changes if they're in the wrong branch.