report() should allow handling of non-200 Stackdriver responses

[calbach]

Current API signature:

report(e, callback), where callback happens on both success or failure (passed XHR error on transaction failure).

In the event that the Stackdriver XHR succeeds with a non-200, e.g. 403 due to API key origin restriction, callback is invoked without parameters, so it looks like a success to the client. This is because XMLHttpRequest.onerror is not dependent on the HTTP status code: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequestEventTarget/onerror

Relevant code:

stackdriver-errors-js/stackdriver-errors.js

Line 146 in f549a50

xhr.onerror = function(e) {

Possibly this method could be changed to accept two callbacks, or else use a Promise. The error path should trigger if the HTTP response status is not a 200.