Phriction - clarify error message when trying to delete already deleted content

Summary: Fixes T7325, T7326, T7328. When you have deleted a document already you have to specify content; this makes this more clear to the user in this specific delete pathway. Also, includes bonus bug fix for T7326 where we weren't moving the title of the wiki page with the rest of the page.

Test Plan: moved a wiki doc and verified it had the title I had specified. tried to delete an already deleted doc via setting the content to blank (i.e. hitting save after making some other edits) and got more clear error UI state

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7328, T7325, T7326

Differential Revision: https://secure.phabricator.com/D11829