Simplify code at a few places

Remove some duplicate checks,
and unnecessary errors.
The default is to scan immediately when the annotator is constructed and the load annotations code here already postpones until after any pending, concurrent scan. If the user passes `dontScan` in the options, we can expect them to obey the contract that `loadAnnotations` shouldn't be called until a scan has been initiated. I would rather remove this code.

I understand we want to get off our fork, but we do not know how long it will take.
The shoot and observer branches also have radically different approaches to anchoring that I disagree with. As far as I am concerned, they are abandoned.

Please, just remove this scan.