moved generic tag rename error message to locale strings

Author: Oaphi

app/controllers/tags_controller.rb

@@ -145,7 +145,7 @@ def rename
       render json: { status: 'success', tag: @tag }
     else
       render json: { status: 'failed',
-                     message: 'Failed to rename the tag.',
+                     message: I18n.t('tags.errors.rename_generic'),
                      tag: @tag },
              status: :bad_request
     end

config/locales/strings/en.tags.yml

@@ -0,0 +1,5 @@
+en:
+  tags:
+    errors:
+      rename_generic: >
+        Failed to rename the tag.

test/controllers/tags_controller_test.rb

@@ -207,7 +207,7 @@ class TagsControllerTest < ActionController::TestCase
     res_body = JSON.parse(response.body)
 
     assert_equal 'failed', res_body['status']
-    assert_no_nil res_body['message']
+    assert_equal I18n.t('tags.errors.rename_generic'), res_body['message']
     tag.reload
     assert_equal tag.name, old_tag_name
   end