openflow: Fix ofp_error.show()
openflow: Fix ofp_error.show()

This function looked up error names wrong, and always or almost always
caused an exception instead of working.  Fixed.

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