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.
Fixing the typos:

2014. nov. 6., cs 21.47-kor-kor, Kristof Csillag <csillag@hypothes.is> 
írta:
> 
> When this happens, only the first call should start a new scan;
> the subsequent calls should simply wait for the existing scan to 
> finish.
> 
=