Fix for upstream changes in kj::Exception

* Change from using Nature/Durability to Type
* Prefer to raise KjException directly instead of Value/RuntimError.
* Will still raise AttributeError appropriately.

test/test_struct.py

@@ -37,9 +37,9 @@ def test_which_builder(addressbook):
     assert bob.employment.which == addressbook.Person.Employment.unemployed
     assert bob.employment.which == "unemployed"
 
-    with pytest.raises(ValueError):
+    with pytest.raises(Exception):
         addresses.which
-    with pytest.raises(ValueError):
+    with pytest.raises(Exception):
         addresses.which
 
 
@@ -71,9 +71,9 @@ def test_which_reader(addressbook):
     bob = people[1]
     assert bob.employment.which == "unemployed"
 
-    with pytest.raises(ValueError):
+    with pytest.raises(Exception):
         addresses.which
-    with pytest.raises(ValueError):
+    with pytest.raises(Exception):
         addresses.which