MM-9688: Better error message for plugin enabling in HA mode (#8433)

* better error message for plugin enabling in HA mode

* wording update

app/plugin.go

@@ -318,6 +318,9 @@ func (a *App) EnablePlugin(id string) *model.AppError {
 	})
 
 	if err := a.SaveConfig(a.Config(), true); err != nil {
+		if err.Id == "ent.cluster.save_config.error" {
+			return model.NewAppError("EnablePlugin", "app.plugin.cluster.save_config.app_error", nil, "", http.StatusInternalServerError)
+		}
 		return model.NewAppError("EnablePlugin", "app.plugin.config.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}
 

i18n/en.json

@@ -3706,6 +3706,10 @@
     "id": "app.plugin.config.app_error",
     "translation": "Error saving plugin state in config"
   },
+  {
+    "id": "app.plugin.cluster.save_config.app_error",
+    "translation": "The plugin configuration in your config.json file must be updated manually when using ReadOnlyConfig with clustering enabled."
+  },
   {
     "id": "app.plugin.deactivate.app_error",
     "translation": "Unable to deactivate plugin"