Bug 24786: Add validation to point of sale

Bug 24786: Add validation to point of sale

The Point of Sale page relied upon serverside validation prior to this
patch; As such with the introduction of the ability to select '--None--'
at the session register level (to require the librarian to pick their
register for every transaction) we could result in a server error page.

This patch introduced clientside validation to add checks for both the
'collected' and 'registerid' form fields.

Test plan:
1/ Select a branch with cash registers available
2/ Select the '--None--' option for the session cash register
3/ Navigate to the point of sale page
4/ Attempt to make a sale without having picked your cash register
5/ Note that the submission was prevented and the fiedl is highlighted
as required in red.
6/ Signoff

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>