SDK: Improve error handling in case state-change fails on a device

Replace the log message with

1. Nothing, if the device is already in the target state
2. Abort and call the completion callback with error otherwise

fairmq/sdk/Topology.h

@@ -150,6 +150,7 @@ class Topology
     ChangeStateCallback fChangeStateCallback;
     std::chrono::milliseconds fStateChangeTimeout;
     bool fShutdown;
+    std::string fStateChangeError;
 
     void WaitForState();
     void AddNewStateEntry(uint64_t senderId, const std::string& state);