I think that currently, PhpSpec does not let the user see any uncaught exception for stuff related to the business logic of phpspec. the only way to see an exception message displayed by the Symfony Console catching would be because of a bug in phpspec breaking something.
As this error is about a invalid user input, it is not a broken PhpSpec execution path. It is a user error, not a PhpSpec one. And we should expect user errors to happen