Allow format change to fail if manual code doesn't parse.

[AnmAtAnm]

• Rename BlockEditorController.onChangeFormat() as .onAttemptChangeFormat().
• Pass in a asynchronous callback to process the view changes.
• Pass false/failure to callback if manual code does not parse. Possibly allow the user to override the change with a confirm(msg).

[AnmAtAnm]

Probably do #168 at the same time. Same spot in the code.