sequencer: do not roll back lockfile unnecessarily

sequencer: do not roll back lockfile unnecessarily

If `commit_lock_file()` or `hold_lock_file_for_update()` fail, there is
no need to call `rollback_lock_file()` on the lockfile. It doesn't hurt
either, but it does make different callers in this file inconsistent,
which might be confusing.

While at it, remove a trailing '.' from a recurring error message.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>