changed to warning when trying to do GW for metals

Since the recent change of raising an exception when trying to do gw for metals broke two tests we here change to only printing a warning. If one tries to do GW for metals the k-point convergence is very poor due to the omitted intraband screening, but in principle the results are not completely off if one just uses a dense enough k-point grid. Therefore it should be sufficient to print a warning for now, and in the future actually enable intraband screening also in GW.