[exceptions] add operator_error, darcy_operator_error

dune/gdt/exceptions.hh

@@ -13,7 +13,15 @@ namespace GDT {
 namespace Exceptions {
 
 
-class prolongation_error : public Dune::Exception
+class operator_error : public Dune::Exception
+{
+};
+
+class prolongation_error : operator_error
+{
+};
+
+class darcy_operator_error : public operator_error
 {
 };