nixos/nextcloud: Account for nix_read_secret refactor in exception msg

Previously, the `nix_read_pwd` function was only used for reading the
`dbpassFile`, however it has since been refactored to handle reading
other secret files too. This fixes the message of the exception that is
thrown in the case that the file is not present so that it no longer
refers specifically to the `dbpass` file.

nixos/modules/services/web-apps/nextcloud.nix

@@ -585,7 +585,7 @@ in {
               function nix_read_secret($file) {
                 if (!file_exists($file)) {
                   throw new \RuntimeException(sprintf(
-                    "Cannot start Nextcloud, dbpass file %s set by NixOS doesn't seem to "
+                    "Cannot start Nextcloud, secret file %s set by NixOS doesn't seem to "
                     . "exist! Please make sure that the file exists and has appropriate "
                     . "permissions for user & group 'nextcloud'!",
                     $file