updated README
set error code in case of "WARN"

if the status is not "OK", return HTTP statuscode 500; this makes it
easier for loadbalancer, so they can just work based on the return code
of the healtchcheck response.
fixed loadbalancer servlet

the status was not reliable, because the clustering status was never
initialized properly. Fixed by reading the repository descriptor on
service startup.

Any changes to the clustering status are then observed by the
clusterAware interface.