route: improve error message

route: improve error message

- fix typ0 in message: NLSMG -> NLMSG
- use strerror() to print a human readable message
- don't print error message if error is ENETUNREACH: it means no route
  found

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20170720082338.1302-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15101.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 20d98427ef37e3b748dbcca2174cd243dcc963dc)