Fix crash on cancelling the File->New dialog

When I want to open a recent window, I often hit ⌘N ⎋ without thinking about it, which causes a crash as AppKit inspects the uninitialized `outError` to find out why PBRepositoryDocumentController didn't create a document.

This commit fixes the crash.  You now get "No document could be created." on hitting File->New, then Cancel - it appears to be impossible to silently return nil from `makeUntitledDocumentOfType`.