Query-frontend worker: put back 'continue' removed in error

Mistake in #4648.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

pkg/querier/worker/frontend_processor.go

@@ -88,6 +88,7 @@ func (fp *frontendProcessor) processQueriesOnSingleStream(workerCtx context.Cont
 			if !grpcutil.IsCanceled(err) {
 				level.Error(fp.log).Log("msg", "error processing requests", "address", address, "err", err)
 				backoff.Wait()
+				continue
 			}
 		}