datapaths: Improve exception
datapaths: Improve exception

We now get a sensible traceback

diogommartins/pox,jacobq/csci5221-viro-project,PrincetonUniversity/pox,jacobq/csci5221-viro-project,pthien92/sdn,denovogroup/pox,chenyuntc/pox,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,PrincetonUniversity/pox,pthien92/sdn,chenyuntc/pox,jacobq/csci5221-viro-project,noxrepo/pox,kavitshah8/SDNDeveloper,carlye566/IoT-POX,diogommartins/pox,kpengboy/pox-exercise,xAKLx/pox,VamsikrishnaNallabothu/pox,kavitshah8/SDNDeveloper,noxrepo/pox,carlye566/IoT-POX,kulawczukmarcin/mypox,kulawczukmarcin/mypox,noxrepo/pox,adusia/pox,kpengboy/pox-exercise,kulawczukmarcin/mypox,waltznetworks/pox,carlye566/IoT-POX,chenyuntc/pox,diogommartins/pox,adusia/pox,PrincetonUniversity/pox,carlye566/IoT-POX,denovogroup/pox,waltznetworks/pox,noxrepo/pox,xAKLx/pox,andiwundsam/_of_normalize,kpengboy/pox-exercise,xAKLx/pox,waltznetworks/pox,kavitshah8/SDNDeveloper,waltznetworks/pox,PrincetonUniversity/pox,MurphyMc/pox,MurphyMc/pox,denovogroup/pox,chenyuntc/pox,denovogroup/pox,MurphyMc/pox,pthien92/sdn,diogommartins/pox,chenyuntc/pox,adusia/pox,waltznetworks/pox,carlye566/IoT-POX,adusia/pox,kavitshah8/SDNDeveloper,kpengboy/pox-exercise,kpengboy/pox-exercise,jacobq/csci5221-viro-project,andiwundsam/_of_normalize,VamsikrishnaNallabothu/pox,andiwundsam/_of_normalize,pthien92/sdn,denovogroup/pox,VamsikrishnaNallabothu/pox,andiwundsam/_of_normalize,xAKLx/pox,PrincetonUniversity/pox,kulawczukmarcin/mypox,MurphyMc/pox,VamsikrishnaNallabothu/pox,pthien92/sdn,MurphyMc/pox,VamsikrishnaNallabothu/pox,xAKLx/pox,diogommartins/pox,adusia/pox