Handle 404s while fetching groups rosters

[sentry-io]

Sentry Issue: LMS-19Y

HTTPError: 404 Client Error: Not Found for url: https://wwu.instructure.com/api/lti/groups/1194647/names_and_roles?limit=100
  File "lms/services/http.py", line 38, in request
    response.raise_for_status()

ExternalRequestError: ExternalRequestError(message=None, request=Request(method='GET', url='https://wwu.instructure.com/api/lti/groups/1194647/names_and_roles?limit=100', body=None), response=Response(status_code=404, reason='Not Found', body='{"errors":[{"message":"The specified resource does not exist."}]}'), validation_errors=None)
(5 additional frame(s) were not displayed)
...
  File "lms/services/roster.py", line 318, in fetch_canvas_group_roster
    roster = self._lti_names_roles_service.get_context_memberships(
  File "lms/services/lti_names_roles.py", line 61, in get_context_memberships
    response = self._make_request(lti_registration, service_url, query)
  File "lms/services/lti_names_roles.py", line 77, in _make_request
    return self._ltia_service.request(
  File "lms/services/ltia_http.py", line 46, in request
    return self._http.request(method, url, headers=headers, **kwargs)
  File "lms/services/http.py", line 40, in request
    raise ExternalRequestError(request=err.request, response=response) from err