Adds meaningful output for invalid state exception of grid functions

Signed-off-by: Santiago Ospina <santiago.ospina@iup.uni-heidelberg.de>

dune/dorie/interface/transport_simulation.cc

@@ -80,9 +80,11 @@ template<typename Traits>
 void TransportSimulation<Traits>::operator_setup ()
 {
   if (!gf_water_flux)
-    DUNE_THROW(Dune::Exception," ");
+    DUNE_THROW(Dune::InvalidStateException,
+        "Pointer to gf_water_flux is invalid!");
   if (!gf_saturation)
-    DUNE_THROW(Dune::Exception," ");
+      DUNE_THROW(Dune::InvalidStateException,
+        "Pointer to gf_saturation is invalid!");
 
   // --- Local Operators ---
   double diff_coeff = inifile.get<double>("parameters.molecularDiffusion");