Add missed plugin error translation (#9131)

app/plugin_statuses.go

@@ -17,7 +17,7 @@ func (a *App) GetPluginStatuses() (model.PluginStatuses, *model.AppError) {
 
 	pluginStatuses, err := a.Plugins.Statuses()
 	if err != nil {
-		return nil, model.NewAppError("GetPluginStatuses", "Unable to get plugin statuses", nil, err.Error(), http.StatusInternalServerError)
+		return nil, model.NewAppError("GetPluginStatuses", "app.plugin.get_statuses.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}
 
 	// Add our cluster ID

i18n/en.json

@@ -3026,6 +3026,10 @@
     "id": "app.plugin.get_plugins.app_error",
     "translation": "Unable to get active plugins"
   },
+  {
+    "id": "app.plugin.get_statuses.app_error",
+    "translation": "Unable to get plugin statuses"
+  },
   {
     "id": "app.plugin.install.app_error",
     "translation": "Unable to install plugin."