extensions-tool/prefs: Log D-Bus errors

We currently ignore any error that may occur when calling the
OpenExtensionPrefs D-Bus method. Right now such an error is highly
unlikely, given that we already checked that we are running under
gnome-shell and the extension in question exists and has prefs.

We'll soon make sure that only one dialog is shown at any time,
which is an error that we can realistically expect, so handle that
properly.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4564

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2013>
(cherry picked from commit 09ed1c533c)

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

@@ -56,6 +56,14 @@ launch_extension_prefs (const char *uuid)
                           NULL,
                           &error);
 
+  if (error)
+    {
+      g_dbus_error_strip_remote_error (error);
+      g_printerr (_("Failed to open prefs for extension “%s”: %s\n"),
+                  uuid, error->message);
+      return FALSE;
+    }
+
   return TRUE;
 }