Better handling of 404 response codes

[pierre]

I had provided a workaround for go-swagger/go-swagger#1929 in #26 and had confirmed it works for 400 and 401 errors.

We just had a report that it might not be enough for some 404 responses (see discussion).

[pierre]

/cc @sbrossie / @andrenpaes / @vlademetal in case you've encountered the issue too?

[hibrid]

I've been trying to reproduce this for a couple of hours and the only way I was able to remotely trigger anything is if the mime type isn't returned (though that isn't a case that should be handled beyond the existing error). In all cases, including 404, where the mime type was text/html, your fix worked correctly