[ex3_b] throw instead of returning random value

exercises/exercise-fluidsystem/fluidsystems/h2omycompressiblecomponent.hh

@@ -228,7 +228,7 @@ public:
             /*!
              * TODO: implement the composition-dependent water density from the exercise sheet.
              */
-            return /*???*/1000.0;
+            DUNE_THROW(Dune::NotImplemented, "Todo: implement composition-dependent density");
         }
         else
         {