Fail Xcode build if Build JS Bundle script phase fails (#39113)

Summary:
Pull Request resolved: #39113

This diff makes Xcode build fail if `Build JS Bundle` script phase fails. Before that the script phase could fail silently resulting in subltle breakages. E.g. we could get seemingly working app that doesn't have proper jsbundle or source maps.
Changelog: [IOS][Fixed] - Fail Xcode build if Build JS Bundle script phase fails.

Reviewed By: motiz88

Differential Revision: D48562229

fbshipit-source-id: 247e271b1ce28061418bb7a8277dcabf3dcf99aa

packages/react-native/scripts/react-native-xcode.sh

@@ -9,7 +9,7 @@
 # and relies on environment variables (including PWD) set by Xcode
 
 # Print commands before executing them (useful for troubleshooting)
-set -x
+set -x -e
 DEST=$CONFIGURATION_BUILD_DIR/$UNLOCALIZED_RESOURCES_FOLDER_PATH
 
 # Enables iOS devices to get the IP address of the machine running Metro