snapshot: use cgit_print_error_page() instead of html_status()

This provides a formatted error response rather than a simple HTTP
error.

Signed-off-by: John Keeping <john@keeping.me.uk>
diff --git a/ui-snapshot.c b/ui-snapshot.c
index cb34f4b..bf4bcd7 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -213,7 +213,7 @@
 	if (!hex && dwim) {
 		hex = get_ref_from_filename(ctx.repo->url, filename, f);
 		if (hex == NULL) {
-			html_status(404, "Not found", 0);
+			cgit_print_error_page(404, "Not found", "Not found");
 			return;
 		}
 		prefix = xstrdup(filename);