sequencer: lib'ify read_and_refresh_cache()

sequencer: lib'ify read_and_refresh_cache()

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

There are two call sites of read_and_refresh_cache(), one of which is
pick_commits(), whose callers were already prepared to do the right
thing given an "error" return from it by an earlier patch, so the
conversion is safe.

The other one, sequencer_pick_revisions() was also prepared to relay
an error return back to its caller in all remaining cases in an
earlier patch.

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