Add PHPDoc type hint to fix PHPStorm inspection error

This is basically a no-op change, just to fix a code inspection error in PHPStorm: it previously complained that "the thrown object must be an instance of the class Exception".

I first noticed this when including Phockito as a composer dependency on a project I'm working on; it's not a huge problem, but it's slightly annoying to have errors on that project (even if only in the /vendor directory).