Show informative errors when attempting to set a recurrence end date on a non-recurring event.

Summary: Fixes T8458, Show informative errors when attempting to set a recurrence end date on a non-recurring event.

Test Plan: Create new event, set recurrence end date via date-picker without setting the "is recurring" checkbox, and attempt to save. Should get error saying there cannot be a recurrence end date on a non-recurring event.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8458

Differential Revision: https://secure.phabricator.com/D13192