Detect ft_stopword_file being unsupported
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
Maniphest Tasks: T2605
Differential Revision: https://secure.phabricator.com/D10342