errors.jsp should return HTTP 503 (Service Unavailable) when there are Johnson Events that prevent the user from accesing the application

We have a set of monitoring tools that ping the JIRA URLs to access the application's health status, although for some classes of errors when a Johnson (/errors.jsp) page is displayed presenting the errors to the user the actual status code returned is HTTP 200, when the expected would be 503 as the service is currently unavailable for the user.
We can use the 'Retry-After' HTTP header to indicate the browser that this is a temporary error and it should retry after a few seconds, this would be the case when a backup is being restored, but in cases when have a JIRA startup problem due to plugin failures for example, it would also indicate a 503, and then we would be able to automatic assess that something is wrong with the instance.

See attached patch.