message: [XLA:Python] Added information about available local backends to RuntimeError.
  When get_local_backend(name=None) is called with `name` that is not in backends, RuntimeError will return names of available ones.
diff: @@ -145,7 +145,8 @@ def get_local_backend(name=None):
  try:
  return backends[name]
  except KeyError:
  - raise RuntimeError('Unknown backend {}'.format(name))
  + raise RuntimeError(
  + 'Unknown backend %s. Available: %s' % (name, list(backends.keys())))
  return list(backends.values())[-1]
  