'Run esptool.py similar to run_esptool, but expect an
error.
Verifies the error is an expected error not an unhandled exception,
and returns the output from esptool.py as a string.'

def run_esptool_error(self, args, baud=None):

    with self.assertRaises(subprocess.CalledProcessError) as fail:
        self.run_esptool(args, baud)
    failure = fail.exception
    self.assertEqual(RETURN_CODE_FATAL_ERROR, failure.returncode)
    return failure.output.decode('utf-8')
