Always return `valid: False` if form validation fails.

The current version of the `validate` view returns `valid: True` if the form is valid or if `final_errors` is empty. This is different from past behaviour and only makes sense when validating a whole form.

When validating individual fields the JS callback (or built in code for p, table and ul types) still needs to execute when the form is invalid even when the individual field being validated has no errors so that the validation errors can be removed from that particular field as the user progresses to the next field.