Use ConnectionErrorObserver, not callbacks, for error handling.

blimp/net/blimp_message_output_buffer.cc

Index: blimp/net/blimp_message_output_buffer.cc
diff --git a/blimp/net/blimp_message_output_buffer.cc b/blimp/net/blimp_message_output_buffer.cc
index 24365eba17fce2bba8ab306ce52c8dda49b4f496..e80a8ae18a6d6e0f8563da4a5ab22d0bcde93f92 100644
--- a/blimp/net/blimp_message_output_buffer.cc
+++ b/blimp/net/blimp_message_output_buffer.cc
@@ -144,17 +144,13 @@ void BlimpMessageOutputBuffer::WriteNextMessageIfReady() {
 }
 
 void BlimpMessageOutputBuffer::OnWriteComplete(int result) {
-  DCHECK_LE(result, net::OK);
-  VLOG(2) << "Write complete, result=" << result;
+  DCHECK_LE(result, 0);
 
+  VLOG(2) << "Write result=" << net::ErrorToString(result);
   if (result == net::OK) {
     ack_buffer_.push_back(std::move(write_buffer_.front()));
     write_buffer_.pop_front();
     WriteNextMessageIfReady();
-  } else {
-    // An error occurred while writing to the network connection.
-    // Stop writing more messages until a new connection is established.
-    DLOG(WARNING) << "Write error (result=" << result << ")";
   }
 }