lockfile: report when rollback fails

lockfile: report when rollback fails

We do not report to the caller when rolling back a lockfile fails, which
will be needed by the reftable compaction logic in a subsequent commit.
It also cannot really report on all errors because the function calls
`delete_tempfile()`, which doesn't return an error either.

Refactor the code so that both `delete_tempfile()` and
`rollback_lock_file()` return an error code.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>