Employees with unlinked Bugzilla accounts

When an employee does not use their Mozilla email for Bugzilla and at the same time, their Bugzilla email is not linked in the phonebook (e.g., https://people.mozilla.org/p/masayuki-nakano). This causes some tools to fail:

2022-07-11 12:07:48,210 - ERROR - Tool no_severity
Traceback (most recent call last):
  File "/home/rm_bot/relman-auto-nag/auto_nag/bzcleaner.py", line 738, in run
    self.send_email(date=date)
  File "/home/rm_bot/relman-auto-nag/auto_nag/bzcleaner.py", line 671, in send_email
    title, body = self.get_email(date)
  File "/home/rm_bot/relman-auto-nag/auto_nag/bzcleaner.py", line 632, in get_email
    data = self.get_email_data(date, bug_ids)
  File "/home/rm_bot/relman-auto-nag/auto_nag/bzcleaner.py", line 623, in get_email_data
    bugs = self.get_bugs(date=date, bug_ids=bug_ids)
  File "/home/rm_bot/relman-auto-nag/auto_nag/bzcleaner.py", line 442, in get_bugs
    timeout=self.get_config("bz_query_timeout"),
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/libmozdata/bugzilla.py", line 176, in wait
    self.wait_bugs()
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/libmozdata/bugzilla.py", line 187, in wait_bugs
    r.result()
  File "/usr/local/lib/python3.7/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/local/lib/python3.7/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/sessions.py", line 653, in send
    r = dispatch_hook('response', hooks, r, **kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/hooks.py", line 31, in dispatch_hook
    _hook_data = hook(hook_data, **kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/libmozdata/bugzilla.py", line 552, in __bugs_cb
    self.bughandler.handle(bug)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/libmozdata/handler.py", line 15, in handle
    self.handler(*args)
  File "/home/rm_bot/relman-auto-nag/auto_nag/bzcleaner.py", line 324, in bughandler
    auto_ni = self.get_mail_to_auto_ni(bug)
  File "/home/rm_bot/relman-auto-nag/auto_nag/scripts/workflow/no_severity.py", line 75, in get_mail_to_auto_ni
    mail, nick = self.round_robin.get(bug, self.date)
  File "/home/rm_bot/relman-auto-nag/auto_nag/round_robin.py", line 174, in get
    nick = self.get_nick(bzmail, pc, cal)
  File "/home/rm_bot/relman-auto-nag/auto_nag/round_robin.py", line 132, in get_nick
    BugzillaUser(user_names=[bzmail], user_handler=handler).wait()
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/libmozdata/connection.py", line 134, in wait
    r.result()
  File "/usr/local/lib/python3.7/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/local/lib/python3.7/concurrent/futures/_base.py", line 384, in __get_result
    raise self._exception
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/sessions.py", line 533, in request
    resp = self.send(prep, **send_kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/sessions.py", line 653, in send
    r = dispatch_hook('response', hooks, r, **kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/hooks.py", line 31, in dispatch_hook
    _hook_data = hook(hook_data, **kwargs)
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/libmozdata/connection.py", line 123, in cb
    res.raise_for_status()
  File "/home/rm_bot/relman-auto-nag/venv/lib/python3.7/site-packages/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 400 Client Error: Bad Request for url: https://bugzilla.mozilla.org/rest/user?include_fields=_default&names=mnakano.birchill%40mozilla.com

Start in auto_nag/round_robin.py, especially get_nick, and trace how the no_severity workflow reaches it from auto_nag/scripts/workflow/no_severity.py. Reproduce the Bugzilla user lookup for an unlinked account and ensure the tool handles that case without the shown HTTP 400 failure.