when i try to join as a DC using invalid credentials i see:
#samba-tool domain join rainer.local DC -UAdministrator --realm=RAINER.LOCAL

Finding a writeable DC for domain 'rainer.local'
Found DC titan_new.rainer.local
Password for [Administrator@RAINER.LOCAL]:
Failed to bind - LDAP error 49 LDAP_INVALID_CREDENTIALS -  <SASL:[GSS-SPNEGO]: NT_STATUS_LOGON_FAILURE> <>
Failed to connect to 'ldap://titan_new.rainer.local' with backend 'ldap': (null)
ERROR(ldb): uncaught exception - None
  File "/usr/lib/python2.7/site-packages/samba/netcmd/__init__.py", line 175, in _run
    return self.run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/samba/netcmd/domain.py", line 552, in run
    machinepass=machinepass, use_ntvfs=use_ntvfs, dns_backend=dns_backend)
  File "/usr/lib/python2.7/site-packages/samba/join.py", line 1082, in join_DC
    machinepass, use_ntvfs, dns_backend, promote_existing)
  File "/usr/lib/python2.7/site-packages/samba/join.py", line 78, in __init__
    credentials=ctx.creds, lp=ctx.lp)
  File "/usr/lib/python2.7/site-packages/samba/samdb.py", line 56, in __init__
    options=options)
  File "/usr/lib/python2.7/site-packages/samba/__init__.py", line 114, in __init__
    self.connect(url, flags, options)
  File "/usr/lib/python2.7/site-packages/samba/samdb.py", line 71, in connect
    options=options)

when i use valid credentials - everything is fine