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