Better errors for bad imports

[jfhbrook]

We should use https://www.python.org/dev/peps/pep-0562/ to make it so that importing event emitters with missing dependencies should raise an ImportError that says something about those missing dependencies. Right now it's an ImportError but just one where the thing isn't there. 🙃

[jfhbrook]

I moved the imports to be namespaced by feature. Problem (eventually) solved!