Adds diagnostic if no return statement found in function

[markwpearce]

If a function explicitly says it will return something, ensure there is a return statement.

This could be improved by checking all branches.

I know there's a BSLint option that basically checks the same thing, but if you don't have a return statement when you say you're going to return a string or an integer, it will crash.

[TwitchBronBron]

Hey, I added some comments about a few more edge cases we need to test for.

[TwitchBronBron]

I agree, this belongs in bsc because it has runtime implications when these are mismatched.