Identify compound short search tokens in the form "xx.yy" as unqueryable in the search UI

Summary:
Ref T12928. The index doesn't work for these, so show the user that there's a problem and drop the terms.

This doesn't fix the problem, but makes the behavior more clear.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12928

Differential Revision: https://secure.phabricator.com/D18254