mediawiki.api: Don't log request error 'abort'

mediawiki.api: Don't log request error 'abort'

Not really an error, but that's outside of our control. This
change doesn't affect the actual Promise object. Callers will
still get this through the error handler.

If the request was aborted by the caller of mw.Api#ajax, there
is no need to log this to the console. Only pollutes it with
garbage.

This was rather visible when browsing with debug=true and e.g.
using search suggestions (and typing faster than requests are
dispatched and completed).

If a caller really wants to log this, they can easily do so from
their error handler.

Change-Id: I7797c833745b452109868675531f808c3d900534