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
assert.ok(/^axios\/[\d.]+[-]?[a-z]*[.]?[\d]+$/.test(res.data), `User-Agent header does not match: ${res.data}`);
        done();
      }).catch(done);
    });
  });
  it('should allow the User-Agent header to be overridden', function (done) {
    server = http.createServer(function (req, res) {
      res.end(req.headers['user-agent']);
    }).listen(4444, function () {
      var headers = { 'UsEr-AgEnT': 'foo bar' }; /
      axios.get('http://localhost:4444/', { headers }).then(function (res) {
        assert.equal(res.data, 'foo bar');
        done();
      }).catch(done);
    });
  });
  it('should allow the Content-Length header to be overridden', function (done) {
    server = http.createServer(function (req, res) {
      assert.strictEqual(req.headers['content-length'], '42');
      res.end();
    }).listen(4444, function () {
      var headers = { 'CoNtEnT-lEnGtH': '42' }; /
      axios.post('http://localhost:4444/', 'foo', { headers }).then(function () {
        done();
      }).catch(done);
    });
  });
  it('should support max content length', async function () {
    server = await startHTTPServer(function (req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');