Not catching 404 not found error from get_attachment

[haroldrubio]

If there is an error where the client cannot find an attachment (see example stack below), the backend returns a 404 but openreview-py does not properly catch the error. This is happening in the Paper Matching Setup process function.

----- Python Traceback -----
    File "/home/openreview/deploy/openreview-py/openreview/openreview.py", line 107, in __handle_response
      response.raise_for_status()
    File "/usr/lib/python3/dist-packages/requests/models.py", line 940, in raise_for_status
      raise HTTPError(http_error_msg, response=self)
  requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://api.openreview.net/attachment?id=a6aESeiUI_&name=upload_affinity_scores