fix: diagnostic try/catch around erroneous JSON response

[russellwheatley]

Description

related to: https://github.com/invertase/flutterfire_cli/issues/30

Type of Change

• [ ] ✨ 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