Add a helpful error if @example() is passed a strategy

[Zac-HD]

Passing a strategy to @example(), as in this StackOverflow question, is probably due to user confusion and usually results in a confusing error message.

It would be nice to check if a strategy was passed as an argument when an error is raised, and raise a more specific exception and message if so. So long as we also set the __cause__, this will have minimal impact even if our guess of the underlying cause is incorrect - and do nothing at all if the test passed.