Properly handle exceptions when permanently storing a video and its thumbnails

[daniil-berg]

The StorageFileController.store_permanently method currently does not catch errors, when .store_file_permanently is called with the video path, but does so and swallows them, what that same method is called with the thumbnail paths.

The entire operation should probably be atomic in that either both video and thumbnails are successfully stored or neither is, but I am not sure how feasible a robust rollback mechanism is in this case.

Whether errors should be propagated or simply logged, it should be done consistently for video and thumbnails.