Am I responsible for unintentional mistakes in my work harvesting grapes, knowing that the employer might not hold me responsible if he knew about them?

If what happened from you was merely an error without negligence or transgression, then you are not liable for what was damaged, because a private employee is not liable except in cases of negligence or transgression. Whether your work was for a specific period or for a specific task within the employer's property, you are considered a private employee. However, if there was negligence or transgression on your part, you must inform the employer and compensate for what was damaged, unless the employer waives their right.