Gracefully handle invalid file paths

`haml-lint` would crash if you passed an invalid file path. Return a
more helpful error message and semantic exit code.

Change-Id: Idb08f9741d9398ad15742432573c913c8a5a5fde
Reviewed-on: http://gerrit.causes.com/40443
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>