message: Add check for valid key URI in AWS KMS.
  The AWS KMS client checks now if the key URI is supported by the client before returning an AEAD primitive.
diff: @@ -134,7 +134,13 @@ class AwsKmsClient(object):
  Returns:
  An AEAD primitive which uses the specified key.
  +
  + Raises:
  + ValueError: If the key_uri is not supported.
  """
  + if not self.does_support(key_uri):
  + raise ValueError('Key URI not supported.')
  +
  key_name = key_uri[len(AWS_KEYURI_PREFIX):]
  return AwsKmsAead(key_name, self.client)
  