Bug 23888: Do not allow invalid vendor id on creating a subscription

Bug 23888: Do not allow invalid vendor id on creating a subscription

It will avoid crash and invalid data when creating/updating a
subscription.

This could have been done with a AJAX query but seems more convenient
this way.

Test plan:
- Create or update a subscription
- In the "Vendor" input try an empty string, a valid vendor's id, and
invalid one.
=> With an empty string you get the existing alert message
=> With a valid id you do not get any messages
=> With an invalid id you are not allowed to go to page 2

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>