RF: more informative message for no setuptools
RF: more informative message for no setuptools

When not running via setuptools, and no nibabel / nisext, we were giving
an undefined name error, which is confusing. Give a better error.

FrancoisRheaultUS/dipy,FrancoisRheaultUS/dipy,mdesco/dipy,samuelstjean/dipy,matthieudumont/dipy,oesteban/dipy,Messaoud-Boudjada/dipy,rfdougherty/dipy,rfdougherty/dipy,StongeEtienne/dipy,sinkpoint/dipy,jyeatman/dipy,nilgoyyou/dipy,villalonreina/dipy,Messaoud-Boudjada/dipy,oesteban/dipy,mdesco/dipy,samuelstjean/dipy,matthieudumont/dipy,demianw/dipy,JohnGriffiths/dipy,beni55/dipy,villalonreina/dipy,StongeEtienne/dipy,demianw/dipy,jyeatman/dipy,samuelstjean/dipy,JohnGriffiths/dipy,beni55/dipy,sinkpoint/dipy,nilgoyyou/dipy