message: Prevent error responses wedging group request concurrency limit
  Fixes
diff: @@ -62,6 +62,11 @@ function limitConcurrency(fn) {
  }
  })
  .then(fn)
  + .catch((err) => {
  + ongoingRequestCount--;
  + checkBacklog();
  + throw err;
  + })
  .then((result) => {
  ongoingRequestCount--;
  checkBacklog();
  