Replace ReleephFieldSpecificationIncompleteException with the CustomField version

Summary: Ref T3718. Releeph has a custom implementation of this exception; a more general version exists in CustomField. Use the more general one. Nothing catches the specific one.

Test Plan: grep

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3718

Differential Revision: https://secure.phabricator.com/D6782