This was we don't get noisy baselines just because a different phase of the compiler reported
the diagnostic.
This helps with Yui's refactoring work to move grammar checks into the type checker.
In effect this fixes#62.
Also
- Changes the error message for get accessors lacking return expressions.
- Actually checks for return expressions instead of return statements for get-accessors.
- Removes fancy quotes.
- Corrects errors in the compiler caught by the new check.
- Simplified `checkAndAggregateReturnTypes` by extracting it out to `visitReturnStatements`.