Throw exception on bad args to SyncRequest

// FREEBIE

libtextsecure/sync_request.js

@@ -8,6 +8,9 @@
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
+        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+            throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
+        }
         this.receiver = receiver;
 
         this.oncontact = this.onContactSyncComplete.bind(this);