Let Mooney-Rivlin throw MathError rather than generic exceptions

Some regularizing algorithms based on exceptions filter the exceptions by type to detect math errors such as ill-formed systems or inputs.

dune/elasticity/materials/mooneyrivlindensity.hh

@@ -88,7 +88,7 @@ public:
     field_type detF = gradient.determinant();
 
     if (detF < 0)
-      DUNE_THROW( Dune::Exception, "det(F) < 0, so it is not possible to calculate the MooneyRivlinEnergy.");
+      DUNE_THROW( Dune::MathError, "det(F) < 0, so it is not possible to calculate the MooneyRivlinEnergy.");
 
     // Add the local energy density
     field_type strainEnergy = 0;