Improve visibility of friendly error messages due to bad configuration

[ydahhrk]

$ sudo jool -bat 192.0.2.1#19221 2001:db8::1#2181
Invalid input data or parameter (System error -7)
$ # ?
$
$ dmesg -t | tail -1
NAT64 Jool ERROR (add_static_route): The transport address (192.0.2.1#19221) does not
belong to the IPv4 pool. Please add it there first.

It would be better if it did this:

$ sudo jool -ba 192.0.2.1#19221 2001:db8::1#2181
The transport address (192.0.2.1#19221) does not belong to the IPv4 pool. Please add it
there first.

Well, granted, if the console is listening to kernel messages, it does something quite acceptable:

$ sudo jool -bat 192.0.2.1#19221 2001:db8::1#2181
NAT64 Jool ERROR (add_static_route): The transport address (192.0.2.1#19221) does not
belong to the IPv4 pool. Please add it there first.
Invalid input data or parameter (System error -7)

But not all consoles do that by default.

This has extremely low priority because

1. Nobody has asked for or bumped it.
2. It has a simple workaround (dmesg).

[kpfleming]

I ran into this problem this past week, and it certainly would have been helpful if the error message was passed back to the userspace tool.

[ydahhrk]

Thanks! Assigned.

I'm not really sure if it's going to be ready by version 3.4.0, but it's a good not-so-introductory exercise for @rolivasnic.

[ydahhrk]

Huh, he already finished this even though I'm still not done tweaking pool4, so I guess this will make it into 3.4.0.