Allow users to unset "Editor", tailor short error messages properly on settings forms

Summary:
Ref T11098.

• Allow "Editor" to be set to the empty string.
• Don't match a validation error to a field unless the actual settings for the field and error match.

Test Plan:

• Tried to set "Editor" to "", success.
• Tried to set "Editor" to "javascript://", only that field got marked "Invalid".

Reviewers: avivey, chad

Reviewed By: chad

Maniphest Tasks: T11098

Differential Revision: https://secure.phabricator.com/D16051