proper error when blacklisting non-existent interface
proper error when blacklisting non-existent interface

intentionet/batfish,dhalperi/batfish,intentionet/batfish,intentionet/batfish,batfish/batfish,dhalperi/batfish,intentionet/batfish,arifogel/batfish,batfish/batfish,arifogel/batfish,dhalperi/batfish,arifogel/batfish,intentionet/batfish,batfish/batfish