Detect ft_stopword_file being unsupported

Summary:
Ref T2605. For old MySQL, this option is not supported. Catch that and tailor the error.

I couldn't find the first version of MySQL which introduced this optino in order to produce a more useful error. I spent about ~10 minutes looking.

Test Plan: Faked the error, survived setup.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2605

Differential Revision: https://secure.phabricator.com/D10342