Handles RPC errors when requesting tablet unload

[keith-turner]

The tablet server group watcher loop will not make progress when it encounters an RPC error on a single tablet server. It should continue communicating with the tablet servers it can inorder to make progress in its loop that assigns and unassigns tablets.

[keith-turner]

Marked this as draft because I have not tested it yet.

[EdColeman]

Why was the logging left at warn? If that's appropriate, it's fine, just wanted to check that reducing the severity level was not overlooked.