Include the oauth setup URL in the error message

When the JIRA issue validation fails because there is no oauth token,
give the user the URL to use to create the trust relationship.

This is a much better user experience than telling the user to go and
find a commit in the web UI with a JIRA issue and click on it.
It would be awesome if we could avoid `ApplicationLinkResponseHandler` and call req.execute() directly! Yes, I think the underlying infrastructure would throw the exception for us. I admit that I haven't poked around the request API too much, and probably grabbed the `ApplicationLinkResponseHandler` bit from some sample code.