Don't respond with a completely blank 404 page.

We respond to all other errors with a simple, 1-line explanation that
you could see when using e.g. curl. The one case of "site is found and
the path is a normal path, but it doesn't exist and the 404 page does
not exist either" was unhandled by accident.

src/pages.go

@@ -202,6 +202,7 @@ func getPage(w http.ResponseWriter, r *http.Request) error {
 				entryPath = notFoundPage
 				continue
 			} else {
+				reader = bytes.NewReader([]byte("not found\n"))
 				break
 			}
 		} else if entry.GetType() == Type_InlineFile {