Add support for cancel reason

[dovca]

I would like the support for a custom reason parameter which a cancelled request would throw.

try {
  const request = MyService.getFoo();
  setTimeout(() => {
    request.cancel(new Error('Request for "foo" timed out!'));
  }, 1000);
  await request;
} catch (err) {
  // err.message === 'Request for "foo" timed out!'
}

Pull request will be provided.