string freeze break request for control-center network panel

[Matthias Clasen <matthias clasen gmail com>]

https://bugzilla.gnome.org/show_bug.cgi?id=646027

is about making the network panel show the same information as the
gnome-shell network menu for devices that cannot be activated. In
particular, we want to indicate if the device cannot be activated
because firmware may be missing. This adds the same 'Firmware missing'
string that was added to gnome-shell a while ago.

The patch has been tested on a system exhibiting the 'firmware
missing' symptom, and works as expected.

I don't think this is super-duper important and could move it to
3.0.1, but on the other hand, it is low risk, and makes the system be
a little more helpful in a broken situation.