Remove unnecessary nil

Empty returns already means `return nil`.
Remove rescue which unexpectedly skips a failed block

Project::VideoHandler#download_video_thumbnail is called just from
workers, not directly affecting the user. If, for some reason the
thumbnail download fail, it used to happen silently. Without it, will
throw an exception and queued to retry list.
Does not let project be updated with thumbnails of invalid video urls
Remove unused action for render video info in json
Protect Project::VideoHandler#video method
Create placeholder image to show when thumbnail upload is in progress
Merge pull request #564 from neighborly/im-remove-rescue-nil

Remove witchcraft of getting video thumbnail while not downloaded yet