The ``request_factory`` (usually provided by aiohttp) callable must now accept ``pre_handler_error``.

Users of the low-level ``aiohttp.Server`` (without an ``Application``) now receive malformed requests via their handler with ``BaseRequest.pre_handler_error`` set to an ``HTTPBadRequest`` and must emit the error response themselves; ``Application`` users are unaffected -- by :user:`Dreamsorcerer`.
