EPHB4 and cerebral cavernous malformation: Identification of the endothelial signalling pathway of CCM3‐DLL4‐Notch‐EphB4‐Erk1/2 constitutes the mechanism of CCM3‐deficiency‐mediated angiogenesis and thus may potentially contribute to new therapeutic concepts in disrupting aberrant angiogenesis in human diseases such as CCM and hyper‐vascularized tumours.