Provide more feedback what went wrong if HipChat service failed test

This MR adds support for displaying the error message during HipChat service test. Before an Error 500 would be displayed with no helpful remarks.

Screenshot:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/10f82eb02db138f9680e69cdb3d3ed82/image.png)

Issue gitlab-com/support-forum#213

See merge request !1144