Api check

This improves the failure mode when a particular Chrome API is not available. Generally, this is because of a missing permissions in the manifest.json. It can take a while to track down the actual issue, so I like the idea of explicitly warning the API client.

That said, I'm not sure this is the best solution, so would like some feedback. @financeCoding @kevmoo  

https://github.com/dart-gde/chrome_gen.dart/issues/36