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.
Would be nice to extract the removal of `@domMapper` into a new `@_removeCurrentAccessStrategy` method or similar. I also feel that the chooseAccessPolicy should be called in the constructor and then when recording a failure and not here.

Then this code could become:]

```coffee
while @domMapper and not @pendingScan
  try
    @pendingScan = this.domMapper.scan()
  catch
     this._removeCurrentAccessStrategy()
...
```

`@pendingScan` also doesn't need to be a property on this object, it can just be a local variable returned by the method that is then used by `loadAnnotations `.

Finally, the comments don't add anything to the code, its safe to remove them.