code: def err_write(self, msg, **kwargs):
          r
          if self._thread_invalid():
              # special case: if a non-main thread writes to stderr
              # i.e. due to an uncaught exception, pass it through
              # without raising an additional exception.
              self.async_call(self.err_write, msg, **kwargs)
              return
          return self.request('nvim_err_write', msg, **kwargs)
docs: r"""Print `msg` as an error message.
  
          The message is buffered (won't display) until linefeed ("\n").