extensions-tool/uninstall: Error out for system extensions

UninstallExtensions() only returns whether the operation was successful,
not why it failed. However we know that only user extensions can be
uninstalled, so check that first to provide a more meaningful error.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391

subprojects/extensions-tool/src/command-uninstall.c

@@ -29,14 +29,27 @@ static gboolean
 uninstall_extension (const char *uuid)
 {
   g_autoptr (GDBusProxy) proxy = NULL;
+  g_autoptr (GVariant) info = NULL;
   g_autoptr (GVariant) response = NULL;
   g_autoptr (GError) error = NULL;
   gboolean success = FALSE;
+  double type;
 
   proxy = get_shell_proxy (&error);
   if (proxy == NULL)
     return FALSE;
 
+  info = get_extension_property (proxy, uuid, "type");
+  if (info == NULL)
+    return FALSE;
+
+  type = g_variant_get_double (info);
+  if (type == TYPE_SYSTEM)
+    {
+      g_printerr (_("Cannot uninstall system extensions\n"));
+      return FALSE;
+    }
+
   response = g_dbus_proxy_call_sync (proxy,
                                      "UninstallExtension",
                                      g_variant_new ("(s)", uuid),
@@ -44,11 +57,6 @@ uninstall_extension (const char *uuid)
                                      -1,
                                      NULL,
                                      &error);
-  if (response == NULL)
-    {
-      g_printerr (_("Failed to connect to GNOME Shell"));
-      return FALSE;
-    }
 
   g_variant_get (response, "(b)", &success);