Raise a better error for trying to create an empty document in Phriction

Summary:
When you delete the content of a document in Phriction, we treat it as an attempt to delete the document.

In the case you're creating the document, we hit an exception trying to delete a document which doesn't exist yet.

Detect this case and raise a better error.

Test Plan: Tried to create an empty document, got a good error. Created a nonempty document. Edited a document to empty to delete it.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1920

Differential Revision: https://secure.phabricator.com/D3728