builtin/mailsplit.c: use error_errno()

builtin/mailsplit.c: use error_errno()

There's one change, in split_mbox(), where an error() without strerror()
as argument is converted to error_errno(). This is correct because the
previous call is fopen (not shown in the context lines), which should
set errno if it returns NULL.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>