Calendar - error better with invalid time values

Summary: Fixes T7665. Unfortunately when the user messes this up its not easy to partially recover so we just reset that time to the default.

Test Plan: set time to "00:00 AM" and got a sensible error.

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7665

Differential Revision: https://secure.phabricator.com/D12174