ctdb-daemon: Exit with error if a database directory does not exist

ctdb-daemon: Exit with error if a database directory does not exist

Since 4.9.0, the log messages can be confusing if a required database
directory does not exist.  Explicitly check for database directories,
logging a clear error and exiting if one is missing.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13696

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Amitay Isaacs <amitay@samba.org>
Autobuild-Date(master): Mon Dec  3 06:56:41 CET 2018 on sn-devel-144

(cherry picked from commit dd7574afd1b2fb6a88defa154bc3d15e94f9ce0d)

Autobuild-User(v4-9-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-9-test): Wed Dec  5 13:01:52 CET 2018 on sn-devel-144