refac: article title rules are the same in the API request and the domain; an invalid title returns 400 instead of 500

test/fakes_common.go

@@ -6,15 +6,27 @@ var errRepositoryFailure = errors.New("repository failure")
 
 type failSwitch struct {
 	method string
+	err    error
 }
 
 func (f *failSwitch) failNext(method string) {
 	f.method = method
+	f.err = nil
+}
+
+func (f *failSwitch) failNextWith(method string, err error) {
+	f.method = method
+	f.err = err
 }
 
 func (f *failSwitch) trigger(method string) error {
 	if f.method == method {
 		f.method = ""
+		if f.err != nil {
+			err := f.err
+			f.err = nil
+			return err
+		}
 		return errRepositoryFailure
 	}
 	return nil