Gracefully handle cases when logging exchange does not exist

Client test suites that test connection recovery by
restarting RabbitMQ ocassionally lead to a situation when
amq.rabbitmq.log in the "/" vhost can be unavailable for a split
second.

Default vhost may or may not exist in general. So handle {error, not_found}
responses from rabbit_basic:publish/4 instead of potentially seriously
polluting logs with confusing exceptions.

While at it, return a sensible value from terminate/2.

Author: michaelklishin

src/rabbit_error_logger.erl

@@ -69,7 +69,7 @@ init([DefaultVHost]) ->
                    name = ?LOG_EXCH_NAME}}.
 
 terminate(_Arg, _State) ->
-    terminated_ok.
+    ok.
 
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
@@ -105,10 +105,11 @@ publish1(RoutingKey, Format, Data, LogExch) ->
 
     Args = [truncate:term(A, ?LOG_TRUNC) || A <- Data],
     Headers = [{<<"node">>, longstr, list_to_binary(atom_to_list(node()))}],
-    {ok, _DeliveredQPids} =
-        rabbit_basic:publish(LogExch, RoutingKey,
-                             #'P_basic'{content_type = <<"text/plain">>,
-                                        timestamp    = Timestamp,
-                                        headers      = Headers},
-                             list_to_binary(io_lib:format(Format, Args))),
-    ok.
+    case rabbit_basic:publish(LogExch, RoutingKey,
+                              #'P_basic'{content_type = <<"text/plain">>,
+                                         timestamp    = Timestamp,
+                                         headers      = Headers},
+                              list_to_binary(io_lib:format(Format, Args))) of
+        {ok, _QPids}  -> ok;
+        {error, _Err} -> ok
+    end.