Bug 8643: Code cleanup

Bug 8643: Code cleanup

Merges "AreMandatoriesNotOk" and new "AreImportantsNotOk" into one
function "AreFieldsNotOk". It can search for mandatory or important
fields depending on the value of it's boolean "mandatory"
parameter.

To test:
This patch should not change current behavior.
Follow the test plan from previous patch and check that saving is
still prevented for unfilled mandatory fields.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>