Complain when a non-void/any 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`.
Removed TODO relating to the task I just fixed.
Minor renaming.
Changed check on annotated functions/getters to only look for return statements.
Changed error message for annotated functions lacking return statements.
Merge pull request #147 from Microsoft/noReturnExpression

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