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
proxyConfig: { hostname: '127.0.0.1', protocol: 'http', port: 80 },
      expectedOptions: { host: '127.0.0.1', protocol: 'http:', port: 80, path: destination }
    }, {
      description: 'both hostname and host -> hostname takes precedence',
      proxyConfig: { hostname: '127.0.0.1', host: '0.0.0.0', protocol: 'http', port: 80 },
      expectedOptions: { host: '127.0.0.1', protocol: 'http:', port: 80, path: destination }
    }, {
      description: 'only host and https protocol',
      proxyConfig: { host: '0.0.0.0', protocol: 'https', port: 80 },
      expectedOptions: { host: '0.0.0.0', protocol: 'https:', port: 80, path: destination }
    }];
    for (const test of testCases) {
      it(test.description, () => {
        const options = { headers: {}, beforeRedirects: {} };
        __setProxy(options, test.proxyConfig, destination);
        for (const [key, expected] of Object.entries(test.expectedOptions)) {
          assert.equal(options[key], expected);
        }
      });
    }
  });
  it('should support cancel', function (done) {
    var source = axios.CancelToken.source();
    server = http.createServer(function (req, res) {
      // c
      source.cancel('Operation has been canceled.');
    }).listen(4444, function () {
      axios.get('http://localhost:4444/', {
        cancelToken: source.token
      }).catch(function (thrown) {