Print human-readable error for "route not found" case.

Submitted by:	vsevolod (initial version)
MFC after:	2 weeks
Sponsored by:	Yandex LLC


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273906 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f