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
});
      }).listen(4000, function () {
        axios.get('http://localhost:4444/', {
          proxy: {
            host: 'localhost',
            port: 4000,
            auth: {
              username: 'user',
              password: 'pass'
            }
          },
          headers: {
            'Proxy-Authorization': 'Basic abc123'
          }
        }).then(function (res) {
          var base64 = Buffer.from('user:pass', 'utf8').toString('base64');
          assert.equal(res.data, 'Basic ' + base64, 'should authenticate to the proxy');
          done();
        }).catch(done);
      });
    });
  });
  context('different options for direct proxy configuration (without env variables)', () => {
    const destination = 'www.example.com';
    const testCases = [{
      description: 'hostname and trailing colon in protocol',
      proxyConfig: { hostname: '127.0.0.1', protocol: 'http:', port: 80 },
      expectedOptions: { host: '127.0.0.1', protocol: 'http:', port: 80, path: destination }
    }, {
      description: 'hostname and no trailing colon in protocol',