Hopefully fix occasional errors when trying to send an initial banner to a frame client. After looking through ws' source, the most likely reason seems to be that the websocket is in CLOSING state. Even if not, we should get a bit more useful log output this time.

lib/units/device/plugins/screen/util/broadcastset.js

@@ -37,4 +37,12 @@ BroadcastSet.prototype.values = function() {
   }, this)
 }
 
+BroadcastSet.prototype.keys = function() {
+  return Object.keys(this.set)
+}
+
+BroadcastSet.prototype.get = function(id) {
+  return this.set[id]
+}
+
 module.exports = BroadcastSet