BUG/MINOR: session: Send a default HTTP error if accept fails for a H1 socket

BUG/MINOR: session: Send a default HTTP error if accept fails for a H1 socket

If session_accept_fd() fails for a raw HTTP socket, we try to send an HTTP error
500. But we must not rely on error messages of the proxy or on the array
http_err_chunks because these are HTX messages. And it should be too expensive
to convert an HTX message to a raw message at this place. So instead, we send a
default HTTP error message from the array http_err_msgs.

This patch must be backported to 2.0 and 1.9.