Raise RuntimeError instead of KeyError if userid was not found on pwchange

``Products.PlonePAS.pas.userSetPassword`` expects a ``RuntimeError`` to be raised if a userid was not found. I'm open for discussion to change that in ``Products.PlonePAS``, but we could change it faster here. 

See: https://github.com/plone/Products.PlonePAS/blob/master/Products/PlonePAS/pas.py#L366