Remove unused Exception from blockdevice.py.
Remove unused Exception from blockdevice.py.

achanda/flocker,jml/flocker,w4ngyi/flocker,LaynePeng/flocker,Azulinho/flocker,1d4Nf6/flocker,wallnerryan/flocker-profiles,runcom/flocker,LaynePeng/flocker,achanda/flocker,w4ngyi/flocker,achanda/flocker,1d4Nf6/flocker,mbrukman/flocker,moypray/flocker,moypray/flocker,agonzalezro/flocker,jml/flocker,adamtheturtle/flocker,moypray/flocker,runcom/flocker,LaynePeng/flocker,1d4Nf6/flocker,wallnerryan/flocker-profiles,adamtheturtle/flocker,jml/flocker,AndyHuu/flocker,Azulinho/flocker,agonzalezro/flocker,runcom/flocker,lukemarsden/flocker,wallnerryan/flocker-profiles,adamtheturtle/flocker,mbrukman/flocker,hackday-profilers/flocker,lukemarsden/flocker,lukemarsden/flocker,AndyHuu/flocker,agonzalezro/flocker,mbrukman/flocker,hackday-profilers/flocker,hackday-profilers/flocker,w4ngyi/flocker,Azulinho/flocker,AndyHuu/flocker