MINOR: proxy: Add references on http replies for proxy error messages

Error messages defined in proxy section or inherited from a default section are
now also referenced using an array of http replies. This is done during the
configuration validity check.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 35ca0ad..62fc435 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -409,6 +409,7 @@
 	char *check_command;			/* Command to use for external agent checks */
 	char *check_path;			/* PATH environment to use for external agent checks */
 	struct buffer *errmsg[HTTP_ERR_SIZE];	/* default or customized error messages for known errors */
+	struct http_reply *replies[HTTP_ERR_SIZE]; /* HTTP replies for known errors */
 	int uuid;				/* universally unique proxy ID, used for SNMP */
 	unsigned int backlog;			/* force the frontend's listen backlog */
 	unsigned long bind_proc;		/* bitmask of processes using this proxy */