The patch for WL 1563 added a new duplicate key error message so that the
key name could be provided instead of the key number. But the error code
for the new message was used even though that did not need to change.
This could cause unnecessary problems for applications that used the old
ER_DUP_ENTRY error code to detect duplicate key errors.
(as part of the auto_increment cleanup of WL#3146; let's not be
sad, that monster push still removed serious bugs):
one problem with INSERT DELAYED (unexpected interval releases),
one with stored functions (wrong auto_inc binlogging).
These bugs were not released.
