print error and/or abort on invalid server definition

While bringing up a new cluster, I managed to fat-finger a sed script and put some lines like this into my config file:

server.1=zookeeper1:2888:2888

This led to a predictable spew of error messages when the client and election components fought over the single port. Since a configuration of this case is always an error, I suggest that it would be sensible to abort the server startup if an entry is found with the same port for both client and election. (Logging the error explicitly without shutting down is less helpful because of how fast the logs pile up.)