libnm: be more accepting for invalid connections from NetworkManager

libnm: be more accepting for invalid connections from NetworkManager

Relax our error checking which will allow us to try harder to
make the best out of whatever NetworkManager sends us.

Also, drop the g_warning(). First, now we really don't expect this
function to fail. And even in that case, raising a g_warning() from
the library is not very friendly to the user of libnm.