client: Prevent socket leaks if socket dir is inaccessible

GVfs fallbacks to session bus if it is not possible to establish
peer-to-peer connection (e.g. inside Flatpak sandbox). However,
the DBus server is not terminated and the socket is leaked. The
named sockets are counted as open files, so it can easily lead to
"Too many open files" errors. Let's fallback to the session bus
immediately if the socket dir is not accessible to prevent the
leaks. This should fix the most common case, when the sockets are
leaked.

Fixes: https://gitlab.gnome.org/GNOME/gvfs/-/issues/542