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.
>  I also feel that the chooseAccessPolicy should be called in the constructor and then when recording a failure and not here.

We can't call it from the constructor yet, because the strategies can be shipped in plugins, which are initiated later then the constructor is run. That's why we only choose a policy when we really have to; ie. when we must scan the document.