message: [tpu]: Have a more helpful error message when no TPU name is specified.
diff: @@ -148,6 +148,9 @@ class TPUClusterResolver(ClusterResolver):
  else:
  tpu = self._envVarFallback()
  + if tpu is None:
  + raise ValueError('Please provide a TPU Name to connect to.')
  +
  self._tpu = compat.as_bytes(tpu) # self._tpu is always bytes
  self._job_name = job_name
  self._credentials = credentials
  