[MINOR] http: take a capture of too large requests and responses

It's hard to prove a request or response is too large if there is no
capture, so let's take a snapshot of those too.
diff --git a/src/proto_http.c b/src/proto_http.c
index a56be38..7f37002 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -2490,6 +2490,8 @@
 			session_inc_http_req_ctr(s);
 			session_inc_http_err_ctr(s);
 			proxy_inc_fe_req_ctr(s->fe);
+			if (msg->err_pos < 0)
+				msg->err_pos = req->l;
 			goto return_bad_req;
 		}
 
@@ -4758,6 +4760,8 @@
 
 		/* too large response does not fit in buffer. */
 		else if (rep->flags & BF_FULL) {
+			if (msg->err_pos < 0)
+				msg->err_pos = rep->l;
 			goto hdr_response_bad;
 		}