Don't warn on unused exports + Do warn on export declarations outside global scope

It's really two separate bugs, but just one patch! It looks to me like export
declarations outside of "global scope" (or whatever the module scope is), is a
syntax error --- traceur will report it as such, so we should probably throw up on it.
yeah, so the code added to handle this is probably not what we want, but it would be a problem for `const` too then, and currently `const` doesn't get stuck in blockscope --- I'm not sure we can add a separate warning for that until it's fixed