nixos/pretix: restart web and worker unit on failure

Found a few instances, where celery intermittently complained about a
misconfigured redis instance and exited.

> redis.exceptions.ResponseError: MISCONF Redis is configured to save RDB
> snapshots, but it's currently unable to persist to disk. Commands that
> may modify the data set are disabled, because this instance is
> configured to report errors during writes if RDB snapshotting fails
> (stop-writes-on-bgsave-error option). Please check the Redis logs for
> details about the RDB error.

nixos/modules/services/web-apps/pretix.nix

@@ -538,6 +538,7 @@ in
           TimeoutStartSec = "15min";
           ExecStart = "${getExe' pythonEnv "gunicorn"} --bind unix:/run/pretix/pretix.sock ${cfg.gunicorn.extraArgs} pretix.wsgi";
           RuntimeDirectory = "pretix";
+          Restart = "on-failure";
         };
       };
 
@@ -559,7 +560,10 @@ in
           "postgresql.service"
         ];
         wantedBy = [ "multi-user.target" ];
-        serviceConfig.ExecStart = "${getExe' pythonEnv "celery"} -A pretix.celery_app worker ${cfg.celery.extraArgs}";
+        serviceConfig = {
+          ExecStart = "${getExe' pythonEnv "celery"} -A pretix.celery_app worker ${cfg.celery.extraArgs}";
+          Restart = "on-failure";
+        };
       };
 
       nginx.serviceConfig.SupplementaryGroups = mkIf cfg.nginx.enable [ "pretix" ];