Takayuki Miyoshi on "[Plugin: Contact Form 7] wpcf7_before_send_mail - Trigger or Add Error Message"

There is no "error_messages" property. Validation is processed in the validate() method of the object, you can find it in includes/classes.php. And, you can't cut into it at the time of wpcf7_before_send_mail hook as validation is processed before sending mail.