Meaningful error codes instead of plain -1

libuavcan/include/uavcan/node/service_server.hpp

@@ -91,7 +91,7 @@ public:
         if (!try_implicit_cast<bool>(callback, true))
         {
             UAVCAN_TRACE("ServiceServer", "Invalid callback");
-            return -1;
+            return -ErrInvalidParam;
         }
         callback_ = callback;