Make failure to converge fatal

src/one-body-sample.org

@@ -430,9 +430,7 @@
           u_saved = u;
   
         if (state_fpi == state_fpi_max)
-          std::cerr << "[ref = " << refinements
-                    << "]: FPI did not converge after "
-                    << state_fpi_max << " iterations" << std::endl;
+          DUNE_THROW(Dune::Exception, "FPI failed to converge");
       }
       <<printOuterProgress>>;