[BUG] AttributeError: 'GSSError' object has no attribute 'message'

[JacobCallahan]

Describe the bug
When an error with the kerberos realm is encountered this library attempts to add an error message to the GSSError's message. However, that message attribute doesn't exist, so an attribute error is raised (See below).

Version-Release number
29.0

To Reproduce
Steps to reproduce the behavior:

1. Set AUTH_METHOD = "krbv"
2. Either don't set a KRB_REALM or set an incorrect one
3. Attempt to run any command that would reach out to a hub
4. See error

Actual behavior

...
  File "/home/jake/Programming/broker/venvbroker/lib64/python3.12/site-packages/bkr/client/commands/cmd_job_list.py", line 269, in run
    self.hub._login()
  File "/home/jake/Programming/broker/venvbroker/lib64/python3.12/site-packages/bkr/common/hub.py", line 114, in _login
    login_method()
  File "/home/jake/Programming/broker/venvbroker/lib64/python3.12/site-packages/bkr/common/hub.py", line 203, in _login_krbv
    ex.message += ". Make sure you correctly set KRB_REALM (current value: %s)." % realm
    ^^^^^^^^^^
AttributeError: 'GSSError' object has no attribute 'message'

Expected behavior
The expected error message would be correctly added to the GSSError message

Additional context
added an IPython embed inside the except clause to inspect the ex object.

In [1]: ex.MESSAGE
Out[1]: 'Major ({maj_stat}): {maj_str}, Minor ({min_stat}): {min_str}'

In [2]: ex.gen_message()
Out[2]: 'Major (...): Unspecified GSS failure.  Minor code may provide more information, Minor (...): Server HTTP/...@....COM not found in Kerberos database'

In [3]: ex.__dict__
Out[3]: 
{'maj_code': ...,
 'min_code': ...,
 'token': None,
 'calling_code': None,
 'routine_code': ...,
 'supplementary_code': None}

In [4]: dir(ex)
Out[4]: 
['MESSAGE',
 '__cause__',
 ... other dunders ...
 '_parse_major_code',
 'add_note',
 'args',
 'calling_code',
 'gen_message',
 'get_all_statuses',
 'maj_code',
 'min_code',
 'routine_code',
 'supplementary_code',
 'token',
 'with_traceback']

[StykMartin]

Changeset merged into release-29. This will be released as part of 29.1.