flow_table: Raise exceptions instead of strings
flow_table: Raise exceptions instead of strings

Raising of strings as exceptions has been deprecated since
Python 1.5 and we probably shouldn't be doing it.

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