Rescue the exception when the logout link is not found

When the logout link is found we get the following error:

[2021-07-07 20:31:19,311] j13/ERROR/locust.user.task: argument of type 'NoneType' is not iterable
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/locust/user/task.py", line 290, in run
    self.execute_next_task()
  File "/usr/local/lib/python3.9/site-packages/locust/user/task.py", line 315, in execute_next_task
    self.execute_task(self._task_queue.pop(0))
  File "/usr/local/lib/python3.9/site-packages/locust/user/task.py", line 327, in execute_task
    task(self)
  File "/Users/jjg/Documents/sources/login/identity-loadtest/load_testing/sp_sign_in.locustfile.py", line 9, in sp_sign_in_load_test
    flow_sp_sign_in.do_sign_in(self)
  File "/Users/jjg/Documents/sources/login/identity-loadtest/load_testing/common_flows/flow_sp_sign_in.py", line 136, in do_sign_in
    logout_link = sp_signout_link(resp)
  File "/Users/jjg/Documents/sources/login/identity-loadtest/load_testing/common_flows/flow_helper.py", line 164, in sp_signout_link
    if "/logout" not in href:
TypeError: argument of type 'NoneType' is not iterable

We should rescue from that exception so that locust can record that as a failure.

We ran into this issue before: https://gsa-tts.slack.com/archives/C010L0SE4E8/p1586795114166400

Start in load_testing/common_flows/flow_helper.py at sp_signout_link, using the traceback to inspect the NoneType failure around the logout-link check. Then review load_testing/common_flows/flow_sp_sign_in.py at do_sign_in and verify that the failure is recorded by Locust rather than escaping as an unhandled exception.