Rebalance: Avoid failing for non-fatal errors (catch-all)

[MB-49079 split from this MB to cover the specific case of merge replica counter failures, which was the subject of CBSE-10499.]

There are multiple phases in index rebalance which can fail with a variety of errors. Current behaviour is to fail rebalance for any error encountered during any of the phases. As failing index service rebalance can block system wide progress, it is not a good idea to fail rebalance for every error.

This is a blanket ticket with the goal of:

a. Investigating all possible errors that can be encountered during rebalance

b. Identify the error cases where index service can continue the rebalance without failing it - may be by taking a work-around path.