[pigeon] Produce a helpful error for bad method return type

[srawlins]

When we encounter an unexpected (or missing) method return type, we now produce a meaningful error instead of a null-pointer exception.

Fixes flutter/flutter#92639

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request improves error handling in Pigeon by providing a more informative error message for methods with unexpected or missing return types, preventing null-pointer exceptions. The changes are accompanied by relevant tests. My review includes a suggestion to fix a typo in a variable name for better code clarity and maintainability.