Improve consistency of error handling in special handler failure path

[sage-mode-hunter]

This patch improves consistency in libucl error handling for special handler failures.

Problem

In the special handler execution path, when a handler fails, libucl unconditionally overwrites parser->err with:

"call for external handler failed"

However, other parts of the parser (e.g. variable expansion and core parser logic) follow a different pattern where an existing error is preserved if already set.

This leads to inconsistent error ownership behavior across modules.

Change

The patch ensures that a previously set error from the special handler is not overwritten:

if (parser->err == NULL) {
    ucl_create_err(&parser->err,
        "call for external handler failed");
}

[sage-mode-hunter]

gentle ping