Resolves: #304

This pr adds a 404 error if the requested path doesn't exist

Testing instructions

• Create an empty folder
• Run wp-now and make sure you add this folder as a path: npx nx preview wp-now start --path=<your-empty-folder>
• Ensure that you don't see any errors in the console
• Ensure that you see 404 File not found in the browser
• Change the URL to something else. eg: /file-not-exist.php.
• Ensure that you still see 404 File not found