Change assertion number in wt_missing_file_errors.js

With the change in WT-5048, WiredTiger will always return WT_TRY_SALVAGE when calling wiredtiger_open() when there is a corrupted metafile. Before, it would miss some cases and return WT_ERROR or WT_PANIC. To merge these WiredTiger changes into the server, we'll need to change an assertion in wt_missing_file_errors.js to respect this change.