AuthFlowResponse response = null;
try
{
    response = await user.StartWithSrpAuthAsync(
    srpRequest: new InitiateSrpAuthRequest
    {
        Password = password
    });
}
catch(Amazon.CognitoIdentityProvider.Model.UserNotConfirmedException e)
{
    // never catches here
}
catch(Amazon.CognitoIdentityProvider.Model.UserNotFoundException e)
{
    // catches here
}