Bug 33781: Terminology: Item already issued to other borrower.

Bug 33781: Terminology: Item already issued to other borrower.

This patch updates a couple of error message strings so that they use
the correct terminology and read well.

"This item is on hold for another borrower." becomes "This item is on
hold for another patron."

"Item already issued to other borrower." becomes "This item is already
checked out to another patron."

To test, apply the patch and log into the OPAC as a patron who has items
checked out which are available for renewal.

In another tab, place a hold on one of those items on behalf of a
different patron.

Back in your OPAC tab, click the "Renew" link next to the title which is
now on hold. The page should reload and show the updated message, "This
item is on hold..."

I was not able to trigger the other message.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>