fix: add underlyingException to FirebaseJsonException when projectId or appId are missing

[Leedwon]

Description

This is a follow up to #354. It improves the error message for misconfigured firebase.json file.

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore

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[Leedwon]

Hi @russellwheatley, I’ve opened a PR for the small improvement we discussed in #354. While testing it locally in my Flutter app, I noticed the underlyingException != null ? '' : underlyingException condition seemed inverted, so I fixed it in a7b0a4f.

However, even with this fix, I couldn't see the exception when building my app. I followed the README and ran dart pub global activate --source="path" . --executable="flutterfire" --overwrite in my fork but still no luck. I also tried using dependency_overrides in my app without success. Could you point me in the right direction on how to properly use the local flutterfire_cli version in my project?

[Leedwon]

I was also thinking about adding a separate message for a case when both projectId and appId is missing, but I'm not sure if it's needed - worst case user will first see projectId is missing and then appId is missing