Improved "not implemented" errors

Improved "not implemented" error message following suggestion from aidge#111 (closed).

include/aidge/operator/MetaOperator.hpp

@@ -118,7 +118,7 @@ public:
     void updateConsummerProducer() override;
     void forward() override;
     void backward() override {
-        assert(false && "not implemented");
+        AIDGE_THROW_OR_ABORT(std::runtime_error, "backward() not implemented yet for a MetaOperator");
     }
 
     inline bool isAtomic() const noexcept override final { return false; }