Still print non-autofix-able errors from arc lint

Summary: `>_<`

Test Plan:
Introduce a lint error in webapp that isn't autofixable (e.g. add too many
newlines to a python file), then run `arc lint`. It should present errors.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12523