Report helpful info via assert if no storage key is found.
Report helpful info via assert if no storage key is found.

Move assert to arc from storage provider factory.

PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs