RF: more informative cython error messages
RF: more informative cython error messages

Say when we can't import cython, and when we have the wrong version.

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