Add check that method exists in Capability.__getattr__.

This makes it play nice with hasattr

test/test_capability_old.py

@@ -49,7 +49,7 @@ def test_client(capability):
 
     assert response.x == '26'
 
-    with pytest.raises(ValueError):
+    with pytest.raises(AttributeError):
         client.foo2_request()
 
     req = client.foo_request()
@@ -110,7 +110,7 @@ def test_simple_client(capability):
     with pytest.raises(ValueError):
         remote = client.foo(i='foo')
 
-    with pytest.raises(ValueError):
+    with pytest.raises(AttributeError):
         remote = client.foo2(i=5)
 
     with pytest.raises(AttributeError):