Show error when saving product without add-on label

[Valentina R]

When saving/updating an appointable product with add-on 'any text' and forgetting to put a label, it doesn't save. It doesn't because it needs the label, but it doesn't say that to the user so people don't know why it's not saving.

Noticed here: https://bizzthemes.com/forums/topic/appointment-products-wont-save/#post-168557

[BookingWP]

Fixed with 3.8.8