samba_backup: fix error handling, back up tdb files properly

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/97#issuecomment-446875356
Comment:
Thanks for trying to improve our locking here.  It certainly did need work. 

Samba 4.9 includes an overhauled backup tool that correctly backs up all TDB and LDB files and obtains the correct locks while doing so, which the old script, even with this patch, couldn't do. 

See ```samba-tool domain backup```