Defines whether to retry and at which consistency level on an unexpected error.

This method might be invoked in the following situations:

1. On a client timeout, while waiting for the server response (see ReadTimeoutMillis).
2. On a socket error (socket closed, etc.).
3. When the contacted host replies with an OVERLOADED error or a SERVER_ERROR.

Note that when such an error occurs, there is no guarantee that the mutation has been applied server-side or not.