Check that at least 2 disks are selected for raid

When configuring RAID, 2 disks are at least needed. Otherwise i.e. the
autoyast.xml template gets wrong values and the installation aborts.
> Could you fix this error message? The string it refers too is definitely not appropriate for here (it has %{node} for substitution).

That's where I copied my error stuff from. I'll fix it.