Add smarter UI for form errors

Summary: Just makes the UI cleaner on full width or dialog forms (and mobile)

Test Plan:
run into a bunch of errors, test mobile breakpoints.

{F281585}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11638