Harden custom date fields against userland adventures

Summary: Users do things like change the type of a field. Currently, we throw when this happens. Instead, recover somewhat-gracefully.

Test Plan:
Created a "string" field, then changed it to a "date" field.

{F35241}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5310