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 756b622 ("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.

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/dns.js b/lib/dns.js
index dd274825b2..3e4b90f6e3 100644
--- a/lib/dns.js
+++ b/lib/dns.js
@@ -34,9 +34,12 @@ function errnoException(err, syscall) {
   if (err === uv.UV_EAI_MEMORY ||
       err === uv.UV_EAI_NODATA ||
       err === uv.UV_EAI_NONAME) {
-    var ex = new Error(syscall + ' ENOTFOUND');
-    ex.code = 'ENOTFOUND';
-    ex.errno = 'ENOTFOUND';
+    err = 'ENOTFOUND';
+  }
+  if (typeof err === 'string') {  // c-ares error code.
+    var ex = new Error(syscall + ' ' + err);
+    ex.code = err;
+    ex.errno = err;
     ex.syscall = syscall;
     return ex;
   }