return 404 on not found errors when fetching a namespace (#72)

glide.yaml

@@ -4,7 +4,7 @@ import:
 - package: github.com/m3db/m3cluster
   version: 53fc512c11e1ed03db2d65dac4c139a3c2ff2eda
 - package: github.com/m3db/m3metrics
-  version: 115abc5764862381dcba3ef30cd0b42026bd69fe
+  version: dcfe68fe112095f688cbaa1b0aa4c4ac65922e84
 - package: github.com/m3db/m3x
   version: a5cc485300c733558a25e1e6c5e9dcf4dbc241c4
 - package: github.com/apache/thrift

service/r2/store/kv/store.go

@@ -478,6 +478,8 @@ func handleUpstreamError(err error) error {
 		return r2.NewConflictError(err.Error())
 	case merrors.ValidationError:
 		return r2.NewBadInputError(err.Error())
+	case merrors.NotFoundError:
+		return r2.NewNotFoundError(err.Error())
 	default:
 		return r2.NewInternalError(err.Error())
 	}

service/r2/store/kv/store_test.go

@@ -117,6 +117,32 @@ func TestUpdateRuleSetVersionMisMatch(t *testing.T) {
 	require.IsType(t, r2.NewConflictError(""), err)
 }
 
+func TestUpdateRuleSetFetchNotFound(t *testing.T) {
+	rsChanges := newTestRuleSetChanges(
+		view.MappingRules{},
+		view.RollupRules{},
+	)
+
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+	mockedStore := rules.NewMockStore(ctrl)
+	mockedStore.EXPECT().ReadRuleSet("testNamespace").Return(
+		nil,
+		merrors.NewNotFoundError("something bad has happened"),
+	)
+
+	storeOpts := NewStoreOptions().SetClockOptions(
+		clock.NewOptions().SetNowFn(func() time.Time {
+			return time.Unix(0, 200)
+		}),
+	)
+	rulesStore := NewStore(mockedStore, storeOpts)
+	uOpts := r2store.NewUpdateOptions().SetAuthor("validUser")
+	_, err := rulesStore.UpdateRuleSet(rsChanges, 1, uOpts)
+	require.Error(t, err)
+	require.IsType(t, r2.NewNotFoundError(""), err)
+}
+
 func TestUpdateRuleSetFetchFailure(t *testing.T) {
 	rsChanges := newTestRuleSetChanges(
 		view.MappingRules{},