Fixes typo of explanation on exception.py
Fixes typo of explanation on exception.py

Something like:
unvavailable -> unavailable

Change-Id: I2f94532862e46d2a1371b819fa6adf2eb9cb3992
Closes-Bug: #1265687

srznew/heat,srznew/heat,gonzolino/heat,ntt-sic/heat,NeCTAR-RC/heat,miguelgrinberg/heat,redhat-openstack/heat,rh-s/heat,miguelgrinberg/heat,pshchelo/heat,maestro-hybrid-cloud/heat,pshchelo/heat,NeCTAR-RC/heat,steveb/heat,cwolferh/heat-scratch,gonzolino/heat,dims/heat,maestro-hybrid-cloud/heat,rdo-management/heat,rh-s/heat,cwolferh/heat-scratch,pratikmallya/heat,openstack/heat,openstack/heat,dims/heat,redhat-openstack/heat,cryptickp/heat,noironetworks/heat,steveb/heat,rdo-management/heat,dragorosson/heat,pratikmallya/heat,jasondunsmore/heat,dragorosson/heat,takeshineshiro/heat,takeshineshiro/heat,jasondunsmore/heat,ntt-sic/heat,cryptickp/heat,noironetworks/heat