If you try to join a domain that can't be found (e.g. misspelt), then you get a TypeError and a nice stack trace.

bin/samba-tool domain join blah.dont.exist dc -UAdministrator%locDCpass1 --targetdir=/tmp/join
INFO 2019-01-16 15:12:40,926 pid:49872 bin/python/samba/join.py #102: Finding a writeable DC for domain 'blah.dont.exist'
ERROR(<class 'TypeError'>): uncaught exception - 'NTSTATUSError' object does not support indexing
  File "bin/python/samba/netcmd/__init__.py", line 184, in _run
    return self.run(*args, **kwargs)
  File "bin/python/samba/netcmd/domain.py", line 699, in run
    backend_store=backend_store)
  File "bin/python/samba/join.py", line 1515, in join_DC
    backend_store=backend_store)
  File "bin/python/samba/join.py", line 103, in __init__
    ctx.server = ctx.find_dc(domain)
  File "bin/python/samba/join.py", line 349, in find_dc
    (domain, error[1]))

Instead, what it should be trying to output is a more helpful message like this:
Failed to find a writeable DC for domain 'blah.dont.exist': The object name is not found.