Kill auto with fire

This literally ruins my day.

If you're working on something, especially in the front-matter, and you type something that doesn't parse... and then type `jekyll --server`

#### Jekyll will not raise and error and just happily runs the server.

No errors, no fails. Just "oh ok. i'll just use the old version of this html in `_site/`.

This blows my mind. if no one else thinks this is a problem i'll close this but i literally hate this.