MEDIUM: proto_htx: Convert all HTTP error messages into HTX

During startup, after the configuration parsing, all HTTP error messages
(errorloc, errorfile or default messages) are converted into HTX messages and
stored in dedicated buffers. We use it to return errors in the HTX analyzers
instead of using ugly OOB blocks.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 7546f01..7394619 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -78,7 +78,7 @@
 void htx_perform_server_redirect(struct stream *s, struct stream_interface *si);
 void htx_server_error(struct stream *s, struct stream_interface *si, int err, int finst, const struct buffer *msg);
 void htx_reply_and_close(struct stream *s, short status, struct buffer *msg);
-
+struct buffer *htx_error_message(struct stream *s);
 
 void debug_hdr(const char *dir, struct stream *s, const char *start, const char *end);
 int apply_filter_to_req_headers(struct stream *s, struct channel *req, struct hdr_exp *exp);