hydra-queue-runner: sleep 5s after handling an exception

instead of immediately calling `readMachinesFiles` again which could
immediately throw another exception again.

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -218,6 +218,7 @@ void State::monitorMachinesFile()
             sleep(30);
         } catch (std::exception & e) {
             printMsg(lvlError, format("reloading machines file: %1%") % e.what());
+            sleep(5);
         }
     }
 }