Make it harder to miss errors and warnings while developing Phabricator

Summary:
If a page generates warnings or errors, you only get a little red dot in
DarkConsole which is hard to see. DarkConsole is also fairly big and there are
plenty of reasons not to leave it open all the time.

Instead, unconditionally show a big message to developers if there are errors or
warnings.

We could make this more sophisticated eventually, but the value is just that you
see it.

Test Plan: Browsed pages with and without warnings, got the right banner state.

Reviewers: nh, btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T734

Differential Revision: https://secure.phabricator.com/D1307