Show error when media format is unsupported (#409)

* fix error about pop

* location-history: add optional callbacks

* set handler on first tick

discovered by @dcposch

* Show error when media format is unsupported

Before this change, the player would just get stuck on the loading
screen forever without notifying the user.

renderer/lib/sound.js

@@ -34,10 +34,6 @@ var sounds = {
     url: 'file://' + path.join(config.STATIC_PATH, 'sound', 'error.wav'),
     volume: 0.2
   },
-  POP: {
-    url: 'file://' + path.join(config.STATIC_PATH, 'sound', 'pop.wav'),
-    volume: 0.2
-  },
   PLAY: {
     url: 'file://' + path.join(config.STATIC_PATH, 'sound', 'play.wav'),
     volume: 0.2