procctl(PROC_REAP_KILL): align error reporting with pgkill(2)

ESRCH from cr_cansignal() means that the target process should be not
visible to the initiator. So do not report pids with ESRCH result.

Noted by: markj
Reviewed by: markj, olce
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D55341