[BUG] http chunking: don't report a parsing error on connection errors

[BUG] http chunking: don't report a parsing error on connection errors

When haproxy parses chunk-encoded data that are scheduled to be sent, it is
possible that the other end is closed (mainly due to a client abort returning
as an error). The message state thus changes to HTTP_MSG_ERROR and the error
is reported as a chunk parsing error ("PD--") while it is not. Detect this
case before setting the flags and set the appropriate flag in this case.