gpgparsemail: Die on parse error, printing errno thing.

* tools/gpgparsemail.c (parse_message): Revert the change.
* tools/rfc822parse.c (transition_to_body): Set ERRNO.
(transition_to_header, insert_header): Likewise.

In the comment of rfc822parse_* functions, it explicitly explained
setting ERRNO on error. For parser errors, it may not have
appropriate ERRNO, in such a case, use ENOENT.

• Fixes-commit: c13e459ffeffb8c5387c44b3c04bb92b7111a75b
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>