Provide nice output of plural form not found error (#147)

Author: d2r2

v2/internal/message_template.go

@@ -2,6 +2,7 @@ package internal
 
 import (
 	"bytes"
+	"fmt"
 
 	"text/template"
 
@@ -38,9 +39,24 @@ func setPluralTemplate(pluralTemplates map[plural.Form]*Template, pluralForm plu
 	}
 }
 
+type pluralFormNotFoundError struct {
+	pluralForm plural.Form
+	messageID  string
+}
+
+func (e pluralFormNotFoundError) Error() string {
+	return fmt.Sprintf("message %q has no plural form %q", e.messageID, e.pluralForm)
+}
+
 // Execute executes the template for the plural form and template data.
 func (mt *MessageTemplate) Execute(pluralForm plural.Form, data interface{}, funcs template.FuncMap) (string, error) {
 	t := mt.PluralTemplates[pluralForm]
+	if t == nil {
+		return "", pluralFormNotFoundError{
+			pluralForm: pluralForm,
+			messageID:  mt.Message.ID,
+		}
+	}
 	if err := t.parse(mt.LeftDelim, mt.RightDelim, funcs); err != nil {
 		return "", err
 	}

v2/internal/message_template_test.go

@@ -1,6 +1,7 @@
 package internal
 
 import (
+	"reflect"
 	"testing"
 
 	"github.com/nicksnyder/go-i18n/v2/internal/plural"
@@ -18,3 +19,15 @@ func TestNilMessageTemplate(t *testing.T) {
 		panic(mt)
 	}
 }
+
+func TestMessageTemplatePluralFormMissing(t *testing.T) {
+	mt := NewMessageTemplate(&Message{ID: "HelloWorld", Other: "Hello World"})
+	s, err := mt.Execute(plural.Few, nil, nil)
+	if s != "" {
+		t.Errorf("expected %q; got %q", "", s)
+	}
+	expectedErr := pluralFormNotFoundError{pluralForm: plural.Few, messageID: "HelloWorld"}
+	if !reflect.DeepEqual(err, expectedErr) {
+		t.Errorf("expected error %#v; got %#v", expectedErr, err)
+	}
+}