Bug 11002: simplify patron mandatory field validation

Bug 11002: simplify patron mandatory field validation

When the pref BorrowerMandatoryField is set and a librarian forget to
fill fields, a js popup appears with the following message:
  "The following fields are mandatory: surname, etc."
The fields are not translatable.

Test plan:
- fill the pref BorrowerMandatoryField with something like:
  title|zipcode|surname|cardnumber|branchcode
- go the members/memberentry.pl?op=add page
- verify you are not allowed to add the patron without filling all
  mandatory fields.

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>