Bug 27064: (QA follow-up) Add error message only if we have one defined

Bug 27064: (QA follow-up) Add error message only if we have one defined

This moves the DestinationEqualsHolding error message addition to if
clause so the value is added to %err hash only when there is actually
an error. Then we only append to the @errmsgloop array the error
message if one was defined.

To test:
 1) Create a hold request to a pickup library where the item currently
 is not
 2) Add transfer to the pickup library using branchtransfers.pl and
 notice without this patch we get and empty message box but with this
 patch we don't get it.

Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>