dns: fix c-ares error reporting regression

The test case from the previous commit exposed a regression in the way
that c-ares errors are reported to JS land.  Said regression was
introduced in commit 756b6222 ("src: add multi-context support").

Fixes the following test failure:

    $ out/Release/node test/simple/test-dns-regress-6244
    util.js:675
      var errname = uv.errname(err);
                       ^
    Error: err >= 0
        at Object.exports._errnoException (util.js:675:20)
        at errnoException (dns.js:43:15)
        at Object.onresolve [as oncomplete] (dns.js:145:19)

lib/dns.js erroneously assumed that the error code was a libuv error
code when it's really a c-ares status code.  Libuv handles getaddrinfo()
style lookups (which is by far the most common type of lookup), that's
why this bug wasn't discovered earlier.