Support document access strategy failover

Detect when scanning with the selected document access strategy fails,
and try to fall back to other strategies.

(This will allow using the Dummy strategy when d-t-m fails for some reason.)

Also, fix TextPosition strategy to avoid crashing without d-t-m.
> Secondly, if the access strategy is not in this list it's going to return -1 which will reset the last item in the list.

That's impossible. 

Currently active access strategy _must_ have been selected from the list of available strategies, so it must be in the array. Nobody else is supposed to mess with that array.

Anyway, I can add a check, if that would make you feel more comfortable. Would it?