Wrong 503-unavailable status code in Distribution API responses

Sometimes we are answered with an HTTP 503-UNAVAILABLE status code when there are unexpected errors within the distribution system.

This status code is misleading since the service is not unavailable, but had an unexpected exception. Maybe a more precise status code would be a 500

However, we need to make sure that we don't break current clients, so this change needs to be discussed.

Cases

1. https://github.com/apache/sling-org-apache-sling-distribution-core/blob/master/src/main/java/org/apache/sling/distribution/servlet/DistributionAgentServlet.java#L74
2. https://github.com/apache/sling-org-apache-sling-distribution-core/blob/master/src/main/java/org/apache/sling/distribution/servlet/DistributionPackageExporterServlet.java#L81
3. https://github.com/apache/sling-org-apache-sling-distribution-core/blob/master/src/main/java/org/apache/sling/distribution/servlet/DistributionPackageExporterServlet.java#L157