Fix a fulltext search issue where finding token length and stopwords could fail

Summary:
Ref T12137. If a database is missing the InnoDB or MyISAM table engines, the big combined query to get both will fail.

Instead, try InnoDB first and then MyISAM.

(I have both engines locally so this worked until I deployed it.)

Test Plan: Faked an InnoDB error like secure, got a MyISAM result.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12137

Differential Revision: https://secure.phabricator.com/D17673