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/index.js

@@ -285,6 +285,11 @@ function dispatch (action, ...args) {
   if (action === 'mediaStalled') {
     state.playing.isStalled = true
   }
+  if (action === 'mediaError') {
+    state.location.back(function () {
+      onError(new Error('Unsupported file format'))
+    })
+  }
   if (action === 'mediaTimeUpdate') {
     state.playing.lastTimeUpdate = new Date().getTime()
     state.playing.isStalled = false