Prevents FatalErrorException being captured twice

See #31

When the stack of handlers is processed (in reverse order) a `FatalErrorException` is passed onto the **second** closure (top of stack) and it will be run as the type hint on `$exception` is `FatalErrorException`. As it returns no response/output, the stack processing continues and it's also run on the **first** closure. As `FatalErrorException` is a sub-type `Exception` it also gets run on that as well.

I will link a pull request for the fix that I have made. I have tested it locally. Let me know if you need any clarification on the above.