net: give better error messages

Add address and/or port to errors where applicable for better reporting.

See #7005
ah found it. When passing an `err` to `uv.errname`, if the err if `ENOTFOUND`, I get this:

```
util.js:739
  var errname = uv.errname(err);
                   ^
Error: err >= 0
    at Error (native)
    at exports._errnoException (util.js:739:20)
    at net.js:941:18
    at Object.asyncCallback [as callback] (dns.js:78:16)
    at Object.onlookup [as oncomplete] (dns.js:91:17)
```