fix: more reliably grab response code in error scenarios

lib/sse.js

@@ -139,7 +139,7 @@ var SSE = function (url, options) {
 
   this._onStreamFailure = function(e) {
     const event = new CustomEvent('error');
-    event.responseCode = this.xhr.status;
+    event.responseCode = e.currentTarget.status;
     event.data = e.currentTarget.response;
     this.dispatchEvent(event);
     this._markClosed();