If invalid number of tickets is provided, the input field simply disappears, no error message occurs

Description of problem:

When creating a booking, you have to insert a number of tickets. If you don't provide the valid value, the input field disappears. No error message is show to the user.

Version-Release number of selected component (if applicable):

WFK2 ER8
EAP6 CR1
both FF 10.0.3 and Google Chrome 17.0

How reproducible:

Always.

Steps to Reproduce:
1. package and deploy Ticket Monster example
2. select an event and venue, click on 'Order Tickets' button
3. choose a section, the input(s) for inserting number of tickets appears
4. provide invalid value (arbitrary letter for instance)
5. click on 'Add tickets' button

Actual results:

The input(s) for number of tickets disappears, the section selector is cleared.

Expected results:

Some error message should be shown to the user, the previously selected section should remain selected.