REST API does not have indication of stopped rebalance (was: couchbase-cli: rebalance reports "SUCCESS" when rebalance is stopped from GUI.)

When a rebalance is started from the CLI, but then stopped (by user) from the GUI the CLI reports "SUCCESS: rebalanced cluster" when the reliance didn't in fact complete. This is misleading, customers may think the rebalance has completed correctly when it didn't.

Expected output:

A message along the lines of: "INFO: rebalance stopped (by user)"

Actual output:

SUCCESS: rebalanced cluster

Steps to reproduce:

1. Existing 2 node cluster, add a third node.
2. Start a rebalance from the CLI:

/opt/couchbase/bin/couchbase-cli rebalance -c localhost:8091 -u Administrator -p password

3. Let it run for a few seconds, then stop from the GUI.