Raise a more tailored exception if transform/thumbnail support is missing for cover images

Summary: If "GD" doesn't support a particular image type, applying a cover image currently goes through but no-ops. Fail it earlier in the process with a more specific error.

Test Plan: Without PNG support locally, dropped a PNG onto a card on a workboard. Got a more useful error.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20328