Proper error message when using ppa with nblocks
nblocks>1 is not compatible with ppa=True, add a more clear error message.
> omegat_GG = E0 * np.sqrt(einv_wGG[1] /
(einv_wGG[0] - einv_wGG[1]))
E IndexError: index 1 is out of bounds for axis 0 with size 1
/home/kuisma/gpaw/gpaw/response/screened_interaction.py:358: IndexError