Network failures result in different err param passed to onError

[jnachtigall]

Steps to reproduce

1. Go to https://react-component.github.io/upload/examples/simple.html
2. Upload a file
3. Notice that as outlined in the example code

      onError(err) {
        console.log('onError', err);
      },

a error message is printed. This error message could be used to show some diagnostics to the user. The error message is assembled via this function in request.js

  xhr.onload = function onload() {
    // allow success when 2xx status
    // see https://github.com/react-component/upload/issues/34
    if (xhr.status < 200 || xhr.status >= 300) {
      return option.onError(getError(option, xhr), getBody(xhr));
    }

    return option.onSuccess(getBody(xhr), xhr);
  };

This is all well and correct.

1. Now in the Network tab of DevTools right click on the previous upload.do request and Block it from the menu (to simulate a network failure).
2. Upload a file - will be marked as Blocked in the Network tab.
3. console.log(err) in onError now outputs a different error object without a message. This is totally different from the previous err parameter and not usable for printing to the user anymore

Looking at the stacktrace this code now comes from

  xhr.onerror = function error(e) {
    option.onError(e);
  };

also in request.js just above xhr.onload

Expected results

Network failures should also pass the same err object to the onError prop as there would be a server message failure, so that it is easier to show such errors to the user.