BUG-1495: fail future when no local RPC is found

Instead of checking state, we return a failed future, which will have a
RpcImplementationUnavailableException as its cause. The current call
site is the remote broker, which is the provider of last resort and it
throws it when it has no foreign system available.

Change-Id: Ib9d8534ca75e60f1867a7c275ee653a70dbd7a00
Signed-off-by: Robert Varga <rovarga@cisco.com>