mark warning as such

[[Imported from SVN: r3201]]

dune/solvers/iterationsteps/multigridstep.cc

@@ -38,7 +38,7 @@ template<class OperatorType, class VectorType, class BitVectorType>
 void MultigridStep<OperatorType, VectorType, BitVectorType>::preprocess()
 {
     if(preprocessCalled)
-        std::cout << "Preprocess has already been called without calling iterate afterwards!" << std::endl;
+        std::cout << "Warning: Preprocess has already been called without calling iterate afterwards!" << std::endl;
     preprocessCalled = true;
 
     int i;