Simplify code at a few places

Remove some duplicate checks,
and unnecessary errors.
This one is an actual behavior change. 

Explanation: the presence of the `domMapper` field indicates that we have already selected a document access strategy. Usually, we do that as the first step of the scanning; but that's not necessarily true. One can also call `_chooseAccessPolicy()`, without actually doing a scan. That would result in the presence of the `domMapper` field, but no scanning data. So in that case, this test would yield the wrong result. (We don't use that case in H.)

The solution is to just call `_scan()` here unconditionally. d-t-m's  `_scan()` is smart enough to determine if it needs to run.