route(8): fix route not found exit code and warn with netlink

Fix route(8) incorrectly returning a zero exit code even when unable to
find the specified route with route -n get <route>.

PR: 274426
Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41882

(cherry picked from commit 2a78083fc2a14af863afb098925b0682698a2838)