Show MySQL exception when unable to connect during setup

Summary: a well-titled diff this be. i feel 'meh' about the change; doesn't
seem to help too much imo.

Test Plan:
edited my custom conf file to have errors -

127.0.0.1 => 127.0.0.2
mysql_user => mysql_users

and for phabricator to be in setup mode. for each error i verified that i liked
the display.

Reviewers: epriestley, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 1129