Make the nbtree page split REDO routine variable names consistent with
_bt_split() (which handles the original execution of page splits).
These names make the code easier to follow by making the distinction
between the original page and the left half of the split clear.  (The
left half of the split page is a temp page that REDO creates to replace
the origpage contents.)

Also reduce the elevel used when adding a new high key to the temp page
from PANIC to ERROR to be consistent.  We already only raise an ERROR
when data item PageAddItem() temp page calls fail.