Downgrade error in object_aclmask_ext() to internal

Downgrade error in object_aclmask_ext() to internal

The "does not exist" error in object_aclmask_ext() was written as
ereport(), suggesting that it is user-facing.  This is problematic:
get_object_class_descr() is meant to be for internal errors only and
does not support translation.

For the has_xxx_privilege functions, the error has not been
user-facing since commit 403ac226ddd.  The remaining users are
pg_database_size() and pg_tablespace_size().  The call stack here is
pretty deep and this dependency is not obvious.  Here we can put in an
explicit existence check with a bespoke error message early in the
function.

Then we can downgrade the error in object_aclmask_ext() to a normal
"cache lookup failed" internal error.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/da2f8942-be6d-48d0-ac1c-a053370a6b1f@eisentraut.org