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
});
  it('should supply a user-agent if one is not specified', function (done) {
    server = http.createServer(function (req, res) {
      assert.equal(req.headers["user-agent"], 'axios/' + axios.VERSION);
      res.end();
    }).listen(4444, function () {
      axios.get('http://localhost:4444/'
      ).then(function (res) {
        done();
      }).catch(done);
    });
  });
  it('should omit a user-agent if one is explicitly disclaimed', function (done) {
    server = http.createServer(function (req, res) {
      assert.equal("user-agent" in req.headers, false);
      assert.equal("User-Agent" in req.headers, false);
      res.end();
    }).listen(4444, function () {
      axios.get('http://localhost:4444/', {
        headers: {
          "User-Agent": null
        }
      }).then(function (res) {
        done();
      }).catch(done);
    });
  });
  it('should throw an error if http server that aborts a chunked request', function (done) {
    server = http.createServer(function (req, res) {
      res.writeHead(200, { 'Content-Type': 'text/plain' });