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/views/player.js

@@ -81,6 +81,7 @@ function renderMedia (state) {
       onplay=${dispatcher('mediaPlaying')}
       onpause=${dispatcher('mediaPaused')}
       onstalling=${dispatcher('mediaStalled')}
+      onerror=${dispatcher('mediaError')}
       ontimeupdate=${dispatcher('mediaTimeUpdate')}
       autoplay>
       ${trackTags}