usage: add get_error_routine() and get_warn_routine()

usage: add get_error_routine() and get_warn_routine()

Let's make it possible to get the current error_routine and warn_routine,
so that we can store them before using set_error_routine() or
set_warn_routine() to use new ones.

This way we will be able put back the original routines, when we are done
with using new ones.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>