Change exception message to something mor generic than contact problems

dune/solvers/iterationsteps/multigridstep.cc

@@ -176,7 +176,7 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::preprocess()
 #endif
 else {
         DUNE_THROW(SolverError, "You can't use " << typeid(*this->basesolver_).name()
-                   << " as a base solver for contact problems!");
+                   << " as a base solver in a MultigridStep!");
     }
 }