chore: Friendly error text when routes are not found

[rdlabo]

Currently, when in Standalone configuration and Route is not found, the following error is displayed

Could not find your routes file. Use the skip-import option to skip importing.

The toolkit knows if a route file is a route file if the naming convention matches /.routes.ts/. However, the file name can be changed at will. So, how about adding a sentence to the error statement such as "routes files are detected based on whether they match /.routes.ts/"?