network: Fix build with NM 0.9.8.x

Team support was only added in the 0.9.9.x branch for the 0.9.10
stable release. It's not available in any releases (beta or stable
ones) yet.

https://bugzilla.gnome.org/show_bug.cgi?id=723769
network: don't warn for invalid settings

g_warning() is for environment errors, not user errors. Not being
able to click OK is enough to understand that something is wrong,
and if not we should highlight the faulty setting, not spam the logs,
expecially in the middle of digitation. Keep a log anyway, for
debugging.

https://bugzilla.gnome.org/show_bug.cgi?id=724312
network: allow an empty gateway address in the connection editor

It appears that this has always been the intention of the code,
but the check was incomplete.

https://bugzilla.gnome.org/show_bug.cgi?id=724312