sequencer: lib'ify read_populate_todo()

sequencer: lib'ify read_populate_todo()

Instead of dying there, let the caller high up in the callchain
notice the error and handle it (by dying, still).

The only caller of read_populate_todo(), sequencer_continue() can
already return errors, so its caller must be already prepared to
handle error returns, and with this step, we make it notice an
error return from this function.

So this is a safe conversion to make read_populate_todo() callable
from new callers that want it not to die, without changing the
external behaviour of anything existing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>