Bug 10306: (QA follow-up) More feedback for admins in koha2marclinks

Bug 10306: (QA follow-up) More feedback for admins in koha2marclinks

As requested by Tomas, this patch does:

[1] Add a js alert when you did not type field,subfield
[2] Print a yellow alert div when the field,subfield is not found.
[3] Bonus: Make it little bit more secure by demanding a POST. (Leaving
    CSRF etc. for another report.)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>