[exceptions] move darcy_operator_error to Darcy Operator

dune/gdt/exceptions.hh

@@ -21,7 +21,7 @@ class prolongation_error : public operator_error
 {
 };
 
-class darcy_operator_error : public operator_error
+class projection_error : public operator_error
 {
 };
 

dune/gdt/operators/darcy.hh

@@ -27,6 +27,15 @@
 
 namespace Dune {
 namespace GDT {
+namespace Exceptions {
+
+
+class darcy_operator_error : public operator_error
+{
+};
+
+
+} // namespace Exceptions
 namespace Operators {