MINOR: http-ana: Add an error message in the txn and send it when defined

It is now possible to set the error message to return to client in the HTTP
transaction. If it is defined, this error message is used instead of proxy's
errors or default errors.
diff --git a/src/http_ana.c b/src/http_ana.c
index 574f6eb..10ee241 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -4597,7 +4597,9 @@
 {
 	const int msgnum = http_get_status_idx(s->txn->status);
 
-	if (s->be->errmsg[msgnum])
+	if (s->txn && s->txn->errmsg)
+		return s->txn->errmsg;
+	else if (s->be->errmsg[msgnum])
 		return s->be->errmsg[msgnum];
 	else if (strm_fe(s)->errmsg[msgnum])
 		return strm_fe(s)->errmsg[msgnum];
@@ -5024,6 +5026,7 @@
 		      ? (TX_NOT_FIRST|TX_WAIT_NEXT_RQ)
 		      : 0);
 	txn->status = -1;
+	txn->errmsg = NULL;
 	*(unsigned int *)txn->cache_hash = 0;
 
 	txn->cookie_first_date = 0;