Log a nicer message when credentials are wrong or not provided

[tmortagne]

Right now if you don't setup any credential you end up with "org.codehaus.jettison.json.JSONException: A JSONObject text must begin with '{' at character 9 of" and the response content which is essentially a jira error page in html telling you "Unauthorized (401)".

1. The plugin should check http response code and indicate that it failed to authenticate
2. Ideally if no credential were provided to the plugin it should ask for them