mediawiki.api: Actually don't log request error 'abort'

173930cb32, once more, with feeling. I'm not sure why the previous
version has ever worked, given that our own code (a few lines above
this, starting from '// Make the AJAX request') is written in such a
way that `code` can never be `'abort'`.

Bug: 65442
Change-Id: I34bcaa362806bf9e80fb40bc92723f85bea1fb64