Clean up another insufficiently-general exception

Summary:
Ref T11044. This is still catching the older exceptions, which are now more general.

If you loaded the web UI without MySQL running, this meant you got a less-helpful error.

Test Plan: Stopped MySQL, loaded web UI, got a more-helpful error.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11044

Differential Revision: https://secure.phabricator.com/D16930