do not log an error for normal oauth2 errors

Such error happens when authorization code is out of date or already used.

src/authentic2_auth_fc/views.py

@@ -92,6 +92,15 @@ def resolve_access_token(authorization_code, redirect_uri, logger):
         response = requests.post(app_settings.token_url, data=data,
                              verify=app_settings.verify_certificate,
                              allow_redirects=False, timeout=3)
+        if response.status_code == 400:
+            try:
+                data = response.json()
+            except ValueError:
+                data = {}
+            logger.warning(u'oauth2 error on access token retrieval '
+                           u'(error=%s, error_description=%s, error_uri=%s)',
+                           data['error'], data['error_description'], data['error_uri'])
+            return
         response.raise_for_status()
     except requests.exceptions.RequestException as e:
         logger.error(u'unable to retrieve access token {}'.format(e))