Clarify reasons for delta node recovery not being possible in UI error message

When the server reports to the UI that delta node recovery is not possible we report to the user the following error message:

Could not Rebalance because requested delta recovery is not possible. You probably added more nodes to the cluster or changed server groups configuration.

We should change this to the following.

Could not rebalance because requested delta recovery is not possible. This may have happened for a number of reasons and is not abnormal. Reasons include: a node was added, server groups were changed, or the failover may have not finished in a way to allow delta recovery to be possible. Please rebalance using full recovery.