Improve behavior of "owner" transaction in "maniphest.edit" endpoint
Summary:
Fixes T10117.
- I accidentally broke setting null to unassign tasks at some point when I added richer validation.
- Raise a better error if the user passes junk.
Test Plan:
- Unassigned a task via API and web UI.
- Reassigned a task via API and web UI.
- Tried to do an invalid assign via API, got a sensible error.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10117
Differential Revision: https://secure.phabricator.com/D14992