Ensure click playback happens at the right time

The code to replay clicks and clean up the handler was called after
processing each thumbnail, instead of just once at the end.
This might have caused many subtle issues such as clicks on any
but the first image not replaying correctly; more problematically,
of there were no MediaViewer-compatible thumbs, the handler was
never cleaned up and the clicks were never replayed.

Besides fixing that, this commit also adds a try..finally wrapper
so that the click replaying is not broken even if there is an error
in the thumb processing code. This might or might not be a good
idea. The internets say that try..finally without a catch causes
errors in IE8, but only if it is not wrapped in another try..catch,
so we are probably fine. (Adding a catch which just rethrows the
error would be an alternative, but it messes up stack traces.)

Change-Id: I2f645762103274c92c15a0d4b595d18d93b08415
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/497
Bug: 64345