Adding error handling when Closure Compiler fails.

When Closure compiler failed, there was no error in the buildout and the result was an empty file. This pull request will add error handling and output the error to the console.

Would be nice if you could integrate this and publish it on PyPi.

Maybe it will also worth it to update to latest version of depswriter and other google related code. I can do it; just let me know if it's worth it.

jeg