Improve exception behavior for storage engine failures

Summary: See T1021. Raise configuration or implementation exceptions immediately. When all engines fail, raise an aggregate exception with details.

Test Plan: Forced all engines to fail, received an aggregate exception. Forced an engine to fail with a config exception, recevied it immediately.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1021

Differential Revision: https://secure.phabricator.com/D2157