Bug 6977: Follow-up patch : catch CloneSubfield errors

Bug 6977: Follow-up patch : catch CloneSubfield errors

A javascript error could occur if the CloneSubfield function failed for
any reason. This would prevent the popup from disappearing.

Example : Importing informations of a UNIF_TITLE authority with multiple
$x subfields in the biblio's 440 field. 440$x isn't repeatable, so an
error occurs when trying to clone it.

http://bugs.koha-community.org/show_bug.cgi?id=6977
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit 6dac7273fbc2eff7805d2a9bda1eb9a275962c00)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>