throw an error when a non-head message i pushed into a stream

lib/streams/core.js

@@ -13,6 +13,14 @@ function NotImplementedError(message) {
    };
 }
 
+function PushError(message) {
+  this.message = message;
+  this.name = "PushError";
+  this.toString = function() {
+    return this.name + " " + this.message;
+  }
+}
+
 function Node(identifier, listenType, emitType) {
   this.identifier = identifier;
   this.listenType = listenType;
@@ -231,10 +239,9 @@ Stream.prototype = {
   push: function _stream_push(messageType, message) {
     /*
      * Push a message down the stream, from the top
-     * @returns a promise that gets resolved when all downstream tasks are done
      */
     if (!(this.heads.hasOwnProperty(messageType) && this.listensTo.hasOwnProperty(messageType))) {
-      deferred.reject("cannot push a non-head message");
+      throw new PushError("cannot push a non-head message");
     }
     let handlers = this.listensTo[messageType] || [];
     //return this._launch_process(handlers, message);
@@ -321,3 +328,5 @@ Stream.prototype = {
 exports.Node = Node;
 exports.createNode = createNode;
 exports.Stream = Stream;
+exports.NotImplementedError = NotImplementedError;
+exports.PushError = PushError;