Give better user notification/warning in case of network split in the cluster

1. Create a cluster with 3 nodes and a bucket with data in it.
2. Enable autofailover with 5 sec as timeout
3. Create a network split in the cluster: On one of the node, block all incoming network traffic from another node in the cluster (example: iptables -A INPUT -s 172.23.98.81 -j DROP)

We don't autofailover the nodes (as expected from the new behaviour from the design document). But a better warning was proposed warning the user of the possible network split. Currently we only log the following message in the UI logs.
Node 'ns_1@172.23.98.81' saw that node 'ns_1@172.23.98.80' went down. Details: [

{nodedown_reason, net_tick_timeout}]
Node 'ns_1@172.23.98.80' saw that node 'ns_1@172.23.98.81' went down. Details: [{nodedown_reason, net_tick_timeout}

]

We should give atleast a warning message dialog like we do when the ip address changes so that the user is more aware of the split (This was suggested as per the design document)