Make sure to maintain errors on source tags after initialization

Previously if you included an empty source url, such as `<source src="" type="video/mp4">`, the player would just show the loading spinner with no errors. This should take care of that (and close #869 again). 
Is there any way `source` could be undefined but `player.tag.initNetworkState_ === 3`? That doesn't seem possible but we still may want to check for source in the second part of this.