Implementation and fixes for Blimp client/engine E2E communication.

blimp/net/blimp_message_demultiplexer.cc

Index: blimp/net/blimp_message_demultiplexer.cc
diff --git a/blimp/net/blimp_message_demultiplexer.cc b/blimp/net/blimp_message_demultiplexer.cc
index 6a1fa64b66a9052b3f8909a8e8d56dc4e782a525..c13fccc7fbe5367950a6f5df71a90be9dbb8220e 100644
--- a/blimp/net/blimp_message_demultiplexer.cc
+++ b/blimp/net/blimp_message_demultiplexer.cc
@@ -37,11 +37,12 @@ void BlimpMessageDemultiplexer::ProcessMessage(
     const net::CompletionCallback& callback) {
   auto receiver_iter = feature_receiver_map_.find(message->type());
   if (receiver_iter == feature_receiver_map_.end()) {
-    DLOG(FATAL) << "No registered receiver for "
+    DLOG(ERROR) << "No registered receiver for "
                 << BlimpMessageToDebugString(*message) << ".";
     if (!callback.is_null()) {
       callback.Run(net::ERR_NOT_IMPLEMENTED);
     }
+    return;
   }
 
   receiver_iter->second->ProcessMessage(std::move(message), callback);