message: Add error message.
diff: @@ -64,6 +64,7 @@ void KellyErrorEstimator::ComputeEstimates()
  flux_space->Update(false);
  auto xfes = solution->ParFESpace();
  + MFEM_ASSERT(xfes->GetVDim() == 1, "Estimation for vector-valued problems not implemented yet.");
  auto pmesh = xfes->GetParMesh();
  this->error_estimates.SetSize(xfes->GetNE());
  