Output strerror from xo_warn

Reported by: bapt
Reviewed by: bapt, ngie

Differential Revision: https://reviews.freebsd.org/D1378


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276273 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f