'Print a group of errors to the stream.
@param flavor: A string indicating the kind of error (e.g. \'TODO\').
@param errors: A list of errors, often L{failure.Failure}s, but
sometimes \'todo\' errors.
@param formatter: A callable that knows how to format the errors.'

def _printResults(self, flavor, errors, formatter):

    for (reason, cases) in self._groupResults(errors, formatter):
        self._writeln(self._doubleSeparator)
        self._writeln(flavor)
        self._write(reason)
        self._writeln('')
        for case in cases:
            self._writeln(case.id())
