[enhancement] Test message when exception is thrown

[electricjones]

It would be great to be able to:

$this->specify("it throws an exception without a default connection", function () {
   // Do something
}, ['throws' => 'Spider\Exceptions\ConnectionNotFoundException', 'message' => 'The exception message']);

And I'm happy to put together a PR if you let me know what syntax/api you would like (ie message, exception message [throws => ['class', 'message']] as well as ['throws' => 'class']

[DavertMik]

thanks again!