Fix bug when exiting Media Viewer while image is loading

Which would cause the image to never appear, even when reopening
Media Viewer. The source of the issue was an uncaught exception in
ThumbnailWidth.

Unfortunately this cannot be covered by E2E because the image
loads too fast in that context, and cucumber/selenium doesn't
have time to catch the placeholder.

Change-Id: I9386f6e857a7974166ddb5eeb7ea731d943eddcf
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/450
Merge "Fix bug when exiting Media Viewer while image is loading"