Progressive Vertebrobasilar Stroke After Bilateral Vertebral Artery Occlusion and Severe Vertebrobasilar Stenosis: Revascularization of the Occluded Left Vertebral Artery After “Drilling in the Dark” with High Tip-Stiffness Microguidewires and Extra-Intracranial Angioplasty and Stenting, with an Unexpected Favorable Outcome