extensions-tool/prefs: Log D-Bus errors

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 09ed1c533c2350b81e2c79f84998e8d3b4c1199a)