RF: fit_tensor no-arg return code to match others
RF: fit_tensor no-arg return code to match others

The other scripts use argparse, which returns error code 2 for no
arguments.  dipy_fit_tensor uses optparse, so has to specify the error
return code.  Make it match the argparse scripts.

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