RF: ignore exception detail in doctest
RF: ignore exception detail in doctest

Exception detail differs in Python 2 and Python 3.

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