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.
This message is slightly misguiding (and `group_error` also is, in the context of `save_node`, which is why I suggested a new message): we don't just fail to save specific settings, but everything.