net: give better error messages

Add address and/or port to errors where applicable for better reporting.

See #7005
Actually, screw using `_extend()`. Let's just do this manually:
```javascript
if (additions && isObject(additions)) {
  var keys = Object.keys(additions);
  for (var i = 0; i < keys.length; i++)
    if (!isNullOrUndefined(keys[i]) && isPrimitive(keys[i]))
      e[i] = keys[i];
}
```
There, much simpler.

I use `isPrimitive()` because I assume no objects would be attached to the error. Would they?