MultigridStep: throw exception when no smoother is provided

Otherwise there will be a less user-friendly null-pointer dereference
in `iterate()`.

dune/solvers/iterationsteps/multigridstep.cc

@@ -98,7 +98,18 @@ void MultigridStep<MatrixType, VectorType, BitVectorType>::preprocess()
             presmoother_[i] = postsmoother_[i] = levelSmoother->second;
         else
         {
+            if (not presmootherDefault_) {
+                DUNE_THROW(
+                    SolverError,
+                    "Falling back to default presmoother on level " << i << ", but no default was set");
+            }
             presmoother_[i]  = presmootherDefault_;
+
+            if (not postsmootherDefault_) {
+                DUNE_THROW(
+                    SolverError,
+                    "Falling back to default postsmoother on level " << i << ", but no default was set");
+            }
             postsmoother_[i] = postsmootherDefault_;
         }
     }