sequencer: start error messages consistently with lower case

sequencer: start error messages consistently with lower case

Quite a few error messages touched by this developer during the work to
speed up rebase -i started with an upper case letter, violating our
current conventions. Instead of sneaking in this fix (and forgetting
quite a few error messages), let's just have one wholesale patch fixing
all of the error messages in the sequencer.

While at it, the funny "error: Error wrapping up..." was changed to a
less funny, but more helpful, "error: failed to finalize...".

Pointed out by Junio Hamano.

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