Complain when a non-void function lacks a return expresson.

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`.