lvmlockd: improve error message about missing global lock

lvmlockd: improve error message about missing global lock

If the VG holding the global lock is removed, we can indicate
that as the reason for not being able to acquire the global
lock in subsequent error messages, and can suggest enabling
the global lock in another VG.  (This helpful error message
will go away if the global lock is enabled in another VG,
or if lvmlockd is restarted.)