Warn about exit in helper

Created by: jaredbeck

In https://github.com/rspec/rspec-core/issues/2508, @JonRowe wondered:

Yes using exit or abort will cause RSpec to quit .. I am wondering if we should print a warning about that happening...

I said:

Yes, please! Such a warning will be very helpful, as in https://github.com/rspec/rspec-rails/issues/2441#issuecomment-1003212668 The current output "No examples found" is misleading.

Then, @pirj asked:

Does anyone want to send a PR?

So, this is what you get. 😅 I'm sure it needs a lot of work. 🤦 If it's at all salvageable, please help me get it cleaned up.