fix minor bug, message variable does not exist

[janeyx99]

When run with --continue-through-error, the script ends with the following error:

Traceback (most recent call last):
  File "run_test.py", line 745, in <module>
    main()
  File "run_test.py", line 741, in main
    print_to_stderr(message)
NameError: name 'message' is not defined
make: *** [macos-compat] Error 1

This PR just changes message to err, which is the intended variable.

[samestep]

lgtm

[facebook-github-bot]

@janeyx99 has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@janeyx99 merged this pull request in 8595416.