Refactor core for better error handling

PR'ing so we can better comment.

First of all this is looking pretty nifty.
We test (as before) if the error is an instance of `errors.Error`, and don't display a stack trace in this case. It was previously `SassDocError`, but since we're already (kind of) in a SassDoc namespace, I thought it was not needed. Also I didn't liked `SassDocWarning`.