rabbit_amqp_management: Use HTTP code 503 for timeout errors

`rabbit_amqp_management` returns HTTP status codes to the client. 503
means that a service is unavailable (which Khepri is while it is in a
minority) so it's a more appropriate code than the generic 500
internal server error.

Author: the-mikedavis

deps/rabbit/src/rabbit_amqp_management.erl

@@ -217,7 +217,7 @@ handle_http_req(<<"PUT">>,
                         DeclaredX;
                     {error, timeout} ->
                         throw(
-                          <<"500">>,
+                          <<"503">>,
                           "Could not create exchange '~ts' in vhost '~ts' "
                           "because the operation timed out",
                           [XName, Vhost])
@@ -299,7 +299,7 @@ handle_http_req(<<"DELETE">>,
             {<<"204">>, null, {PermCache, TopicPermCache}};
         {error, timeout} ->
             throw(
-              <<"500">>,
+              <<"503">>,
               "failed to delete exchange '~ts' due to a timeout",
               [XNameBin])
     end;