merge-ort-wrappers: make printed message match the one from recursive

When the index does not match HEAD, the merge strategies are responsible
to detect that condition and abort. The merge-ort-wrappers had code to
implement this and meant to copy the error message from merge-recursive
but deviated in two ways, both due to the message in merge-recursive
being processed by another function that made additional changes:
* It added an implicit "error: " prefix
* It added an implicit trailing newline
We can get these things by making use of the error() function.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>