[MEDIUM] errorloc now checked first from backend then from frontend

It is now possible to define an errorloc in the backend as well as
in the frontend. The backend's will be used first, and if undefined,
then the frontend's will be used instead. If none is used, then the
original error messages will be used.
diff --git a/include/proto/httperr.h b/include/proto/httperr.h
index 24a0cd1..749e427 100644
--- a/include/proto/httperr.h
+++ b/include/proto/httperr.h
@@ -25,12 +25,14 @@
 #include <types/httperr.h>
 
 extern const int http_err_codes[HTTP_ERR_SIZE];
-extern const char *http_err_msgs[HTTP_ERR_SIZE];
+extern struct chunk http_err_chunks[HTTP_ERR_SIZE];
 extern const char *HTTP_200;
 extern const char *HTTP_302;
 extern const char *HTTP_303;
 extern const char *HTTP_401_fmt;
 
+struct chunk *error_message(struct session *s, int msgnum);
+
 #endif /* _PROTO_HTTPERR_H */
 
 /*