lp1012237 HOLD_ITEM_CHECKED_OUT display

Added a definition to the HOLD_ITEM_CHECKED_OUT event in ils_events.xml.
Trying to be generic between the opac and the staff client the message
reads, "The item you have attempted to place on hold is already checked
out to the requestor."

[LFW: I made a tiny spelling change to be consistent with other
appearances in Evergreen of the term "requestor".]

Signed-off-by: Garry Collum <gcollum@gmail.com>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>