Rethink what axe does when context selectors don't match, or match inappropriate elements

Currently:

- If axe is passed an invalid selector, it defaults to test the entire page
- If passed a selector that doesn't match anything it throws
- If passed a frame selector that matches an element that isn't a frame, it logs

This is kind of inconsistent and confusing. I think axe should give devs better control of what to do when it encounters problems when building the context.