Currently, if you use axios with async / await, you get a stack trace like this:

```
Request failed with status code 500
    at createError (/Users/rmenezes/code/seedfi/web/node_modules/axios/lib/core/createError.js:16:15)
    at settle (/Users/rmenezes/code/seedfi/web/node_modules/axios/lib/core/settle.js:17:12)
    at IncomingMessage.handleStreamEnd (/Users/rmenezes/code/seedfi/web/node_modules/axios/lib/adapters/http.js:237:11)
    at IncomingMessage.emit (events.js:208:15)
    at IncomingMessage.EventEmitter.emit (domain.js:471:20)
    at endReadableNT (_stream_readable.js:1154:12)
    at processTicksAndRejections (internal/process/task_queues.js:77:11
```

It doesn't show you much about *what* called Axios., which makes debugging quite difficult!

This is because `settle` calls `createError`:
https://github.com/axios/axios/blob/master/lib/core/settle.js#L12-L24

Which creates a new error with a brand new stack trace:
https://github.com/axios/axios/blob/master/lib/core/createError.js#L15-L18

Perhaps we can fix this by creating an error before the request goes out and stitching it to the new error's stack trace?
test/unit/adapters/http.js
};
        http.get(opts, function (res) {
          var body = '';
          res.on('data', function (data) {
            body += data;
          });
          res.on('end', function () {
            response.setHeader('Content-Type', 'text/html; charset=UTF-8');
            response.setHeader('Location', res.headers.location);
            response.end(body);
          });
        });
      }).listen(4000, function () {
        axios.get('http://localhost:4444/').then(function(res) {
          assert.equal(res.status, 200);
          assert.equal(proxyUseCount, 1);
          done();
        }).catch(done);
      });
    });
  });
  it('should not use proxy for domains in no_proxy', function (done) {
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
      res.end('4567');
    }).listen(4444, function () {
      proxy = http.createServer(function (request, response) {
        var parsed = url.parse(request.url);
        var opts = {
          host: parsed.hostname,