Feature and motivation

Right now, Selenium Manager is called when a driver isn't specified or found on PATH. We only support 3 architectures, though. We should add code to explicitly check the user's architecture in preparation for additional additional architectures and throw an error until the architecture is actually supported.

This isn't as much of a problem while the Manager is in a "fall back" mode, but it can cause unexpected/confusing errors especially once we stop hiding them.