As the programmer, you can identify the error and then output any error you wish. I would not recommend doing this by changing code in recaptchalib.php, however.
In
your code, just detect the error with an if() and then echo whatever message you like:
if( $recaptcha_response->error == 'incorrect-captcha-sol') {
echo 'Incorrect Captcha. Please enter the characters you see below.';
}