Added error handling to prevent a crash

If an error is returned by a http request, there is no response object which will lead to a crash with an error like "could not get statusCode of undefined".
To prevent this crash, we need an error handler.

An error on a request can occur when per example the jira server is not available. (e.g. no Internet Connection)

Can somebody please update the docs. I don't know how to do that.