Handle 404s while fetching groups rosters

Sentry Issue: [LMS-19Y](https://hypothesis.sentry.io/issues/6125756509/?referrer=github_integration)

```
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
```