RPC usability: throw a checked exception when no implementations are registered

When an RPC call fails to find a target, we throw an IllegalStateException. Create a new checked exception and propagate it as close to caller as possible without converting it into runtime exception.

For Helium: this should end up being the cause of an ExecutionException, if at all possible.

For Lithium: this should be a subclass of an exception declared as the CheckedFuture result.