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/dispatcher.js

@@ -20,7 +20,7 @@ function dispatcher (...args) {
   var json = JSON.stringify(args)
   var handler = _dispatchers[json]
   if (!handler) {
-    _dispatchers[json] = (e) => {
+    handler = _dispatchers[json] = (e) => {
       // Don't click on whatever is below the button
       e.stopPropagation()
       // Don't regisiter clicks on disabled buttons