`g_dbus_proxy_call_sync` will return NULL and set the recoverable
`error` in case of failure.
We didn't catch this previously so reading the variant from the return
value caused a CRITICAL because a user didn't have the `week-start-day`
setting (introduced in 50.alpha) and the returned `GVariant` was NULL.
Now, we check whether en error is set after calling
`g_dbus_proxy_call_sync` and propagate it to the calling method so it
can gracefully handle it.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/862>
mindonwarp
81fa0f29c4