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
Error reporting is useful when used interactively, but often undesirable
when used in scripts. Account for that with a common --quiet option,
which is more convenient than redirection stderr to /dev/null.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391
The gnome-extensions tool code is really independent from the rest of the
code base, and could be used either as part of the gnome-shell build or as
stand-alone project (for example for the extension-ci docker image).
We can actually support both cases by moving the code to a subproject.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/877
