Add error check in Python interface when setting the functional.
Add error check in Python interface when setting the functional.

dftlibs/xcfun,bast/xcfun,bast/xcfun,robertodr/xcfun,dftlibs/xcfun,robertodr/xcfun,robertodr/xcfun,robertodr/xcfun,bast/xcfun,dftlibs/xcfun,dftlibs/xcfun,bast/xcfun