Quick fix for black screen of death

This fixes three issues:
* the overlay way multiplied during history navigation, but only
  the last one was cleaned up on exit
* the overlay was not cleaned up when loading a #mediaviewer URL
  with an invalid file name
* the overlay was not cleaned up when there was an error creating
  the viewer object

This is a quick fix, and does not change the fact that
bootstrapping is an unmaintainable mess which should be cleaned up.

Change-Id: I2838e5e709e7c7308b0a5b27eca24a2584d0a01e
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/414
Merge "Quick fix for black screen of death"