Code review comment for lp://staging/~sylvain-pineau/checkbox-core/bug983035

I just thought of a solution to solve the problem of having create_file somehow return an errno: instead of returning None when an error occurs, we could return the negative errno:

  file = create_file(path)
  if file < 0:
    logging.error("Failed to open file '%s'; errno = %d", path, -file)

This will work when create_file returns an actual object, so the code is safe. The only problem is that the value assigned to file is either an object or a negative integer. This might sound strange but it's not that much diffferent from returning None. For example, both a negative integer and None will raise an AttributeError when attempting to call file.read() when create_file fails. What do you think?