libnm: change error message about routing-rules without priority

When you do

  $ nmcli connection modify "$PROFILE" +ipv4.routing-rules 'uidrange 1000-1000 lookup 12345'
  Error: failed to modify ipv4.routing-rules: rule is invalid: invalid priority.

That message seems confusing. Reword.

src/libnm-core-impl/nm-setting-ip-config.c

@@ -2684,7 +2684,7 @@ nm_ip_routing_rule_validate(const NMIPRoutingRule *self, GError **error)
         g_set_error_literal(error,
                             NM_CONNECTION_ERROR,
                             NM_CONNECTION_ERROR_INVALID_PROPERTY,
-                            _("invalid priority"));
+                            _("missing priority"));
         return FALSE;
     }