Bug 1058853 - Don't break the gallery if we have corrupt images

Rather than passing null to the metadataCallback, isn't there an error callback to call instead? If the image is unusable, we want to reject it with the mechanism that mediadb already provides, not invent a new null metadata mechanism.

Also, if we have to do this, I think that a console.error() call would be appropriate here to explain why the image is being skipped.