_expection.py: Centralise exception declarations

buildgrid/_exceptions.py

buildgrid/server/_exceptions.py

buildgrid/server/actioncache/service.py

buildgrid/server/bots/instance.py

buildgrid/server/bots/service.py

buildgrid/server/cas/instance.py

buildgrid/server/cas/service.py

buildgrid/server/execution/instance.py

buildgrid/server/execution/service.py

buildgrid/server/operations/instance.py

buildgrid/server/operations/service.py

buildgrid/server/referencestorage/service.py

buildgrid/server/referencestorage/storage.py