Inserted warning about vdW-DF + nspin>1 cases; fixed inconsistency in check for such cases, but generally now produce an error anyway, as this uncontrolled approximation should not be used

git-svn-id: http://qeforge.qe-forge.org/svn/q-e/trunk/espresso@11103 c92efa57-630b-4861-b058-cf58834340f0