[mustache_template] Improve error throwing

[augustocesarperin]

Addresses error handling issues flagged in the Gemini review of mustache_template.

Changes:

• Added missing throw before _error() calls in lambda_context.dart
• Changed Exception to StateError for unreachable code in parser.dart

Fixes flutter/flutter#174938

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[gemini-code-assist]

Code Review

This pull request effectively addresses error handling issues by adding missing throw statements in LambdaContext and replacing Exception with the more appropriate StateError for unreachable code in Parser. The changes are correct and improve the code's robustness. I have a few minor suggestions to ensure consistency in error message formatting.

[stuartmorgan-g]

Just changelog nits; thanks for this cleanup!

This could be considered a breaking change, since calls that didn't throw before now do, so we could be conservative and make this a major version change, but since it only affects incorrect usage, and thus hopefully won't affect any actual clients, I think we can treat is as a bugfix. (If that turns out to be incorrect, and a non-trivial number of people are broken by it, we can retract it.)