route: warn on IPv4 routes installation when no IPv4 is configured

route: warn on IPv4 routes installation when no IPv4 is configured

Same as already happens for IPv6, it is useful for the user to throw a
warning when an IPv4 route is about to be installed and the tun interface
has no IPv4 configured.

The twin message for IPv4 is adapted to have the same format.

The warning is not fatal, becuase the route might actually be external
to the tun interface and therefore it may still work.

At the same time, modify the error message used for a route
installation failure to explicitly mention "IPv4" since this it is
used in the IPv4 code path only.

Trac: #208
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200530000600.1680-6-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19946.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>