Better error message when missing entry file (#29012)

Summary:
If the entry file has been renamed (e.g., to `index.ts`), the script currently fails silently and prints out that the bundle file does not exist.

This change will print the correct error message instead.

## Changelog

[iOS] [Fixed] - Better error message when missing entry file
Pull Request resolved: https://github.com/facebook/react-native/pull/29012

Test Plan: N/A

Reviewed By: hramos

Differential Revision: D22109657

Pulled By: TheSavior

fbshipit-source-id: edd7b6f88becc954ee41ed01ae43efb4481adc95