Raise human-readable error messages for overlong Phame blog titles and subtitles

Summary:
Fixes T11358. Entering a too-long title/subtitle currently raises an unfriendly (database-level) error.

Raise a friendlier error.

Test Plan: {F1731533}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11358

Differential Revision: https://secure.phabricator.com/D16313