nixos/keycloak: pass --verbose to starting keycloak

This sets the exception handler to show the full exception on startup.
We don't think it does anything else, with respect to logging, for
instance. Everything else can be configured in the config file, and this
is plain reasonable to simply always enable in our view.

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

@@ -663,7 +663,7 @@ in
             '' + ''
               export KEYCLOAK_ADMIN=admin
               export KEYCLOAK_ADMIN_PASSWORD=${escapeShellArg cfg.initialAdminPassword}
-              kc.sh start --optimized
+              kc.sh --verbose start --optimized
             '';
           };