[BUG]: 502 Bad Gateway during credentials process

[thinkjrs]

Describe the bug

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/gunicorn/arbiter.py", line 583, in spawn_worker
    worker.init_process()
  File "/usr/local/lib/python3.8/site-packages/gunicorn/workers/gthread.py", line 92, in init_process
    super().init_process()
  File "/usr/local/lib/python3.8/site-packages/gunicorn/workers/base.py", line 119, in init_process
    self.load_wsgi()
  File "/usr/local/lib/python3.8/site-packages/gunicorn/workers/base.py", line 144, in load_wsgi
    self.wsgi = self.app.wsgi()
  File "/usr/local/lib/python3.8/site-packages/gunicorn/app/base.py", line 67, in wsgi
    self.callable = self.load()
  File "/usr/local/lib/python3.8/site-packages/gunicorn/app/wsgiapp.py", line 49, in load
    return self.load_wsgiapp()
  File "/usr/local/lib/python3.8/site-packages/gunicorn/app/wsgiapp.py", line 39, in load_wsgiapp
    return util.import_app(self.app_uri)
  File "/usr/local/lib/python3.8/site-packages/gunicorn/util.py", line 358, in import_app
    mod = importlib.import_module(module)
  File "/usr/local/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/application/app.py", line 27, in <module>
    import utilities
  File "/application/utilities.py", line 6, in <module>
    import pycmc
  File "/usr/local/lib/python3.8/site-packages/pycmc/__init__.py", line 7, in <module>
    from . import credentials_manager
  File "/usr/local/lib/python3.8/site-packages/pycmc/credentials_manager.py", line 52, in <module>
    credentials.Update()  # first refresh our credentials
  File "/usr/local/lib/python3.8/site-packages/pycmc/credentials.py", line 103, in Update
    fetched = FetchAccessToken()
  File "/usr/local/lib/python3.8/site-packages/pycmc/credentials.py", line 182, in FetchAccessToken
    response.raise_for_status()
  File "/usr/local/lib/python3.8/site-packages/requests/models.py", line 941, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 502 Server Error: Bad Gateway for url: https://api.chartmetric.com/api/token

To Reproduce
See above stacktrace, however, it's obvious that something in the update process caused the error. ⚠️ The error only occurred for two minutes on November 25th between 9:15 and 9:16 CST.

Expected behavior
No error.

Additional context
The credentials update process is byzantine and could use an update. Possibly related to #25 !