Need meaningful error message when DDL commands are run during GSI rebalance

Steps to reproduce:

1) Create a 4 node cluster : 1 kv, 2 indexer, 1 n1ql

2) Create multiple secondary indexes.

3) Remove one of the indexer nodes and start rebalance.

4) While step 3 is in progress, run DDL(like create index) from the n1ql node. Query results in "Unexpected server error".

A meaningful error message indicating that a GSI rebalance is in progress would be helpful.