ACPICA: Update an error message for a duplicate table

In this case, the exception AE_ALREADY_EXISTS is more
appropriate.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>