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: #29012

Test Plan: N/A

Reviewed By: hramos

Differential Revision: D22109657

Pulled By: TheSavior

fbshipit-source-id: edd7b6f88becc954ee41ed01ae43efb4481adc95

Author: petrbela

scripts/react-native-xcode.sh

@@ -78,9 +78,14 @@ if [[ "$ENTRY_FILE" ]]; then
   # Use ENTRY_FILE defined by user
   :
 elif [[ -s "index.ios.js" ]]; then
-   ENTRY_FILE=${1:-index.ios.js}
- else
-   ENTRY_FILE=${1:-index.js}
+  ENTRY_FILE=${1:-index.ios.js}
+else
+  ENTRY_FILE=${1:-index.js}
+fi
+
+if [[ $DEV != true && ! -f "$ENTRY_FILE" ]]; then
+  echo "error: Entry file $ENTRY_FILE does not exist. If you use another file as your entry point, pass ENTRY_FILE=myindex.js" >&2
+  exit 2
 fi
 
 if [[ -s "$HOME/.nvm/nvm.sh" ]]; then