Fix errcheck issue in oauth.go (#28795)

server/i18n/en.json

@@ -2360,6 +2360,10 @@
     "id": "api.oauth.get_access_token.bad_grant.app_error",
     "translation": "invalid_request: Bad grant_type."
   },
+  {
+    "id": "api.oauth.get_access_token.bad_request.app_error",
+    "translation": "invalid_request: Bad request."
+  },
   {
     "id": "api.oauth.get_access_token.credentials.app_error",
     "translation": "invalid_client: Invalid client credentials."