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.
I wonder if it would be a good idea to catch `ResponseException` and re-throw an un-wrapped `CredentialsRequiredException`. That way, the calling method would only have to catch `CredentialsRequiredException` to detect auth errors rather than both it and `ResponseException`. This could allow us to remove the nearly duplicated `CredentialsRequiredException` logic in `YaccServiceImpl`, and the 2 supporting test cases that are pretty similar.