Expose warnings during master-failover

Expose warnings during master-failover

During master failover, there are some situations where problems
occur but the best thing to do is to carry on. These problems
are logged using the usual mechanism. However, a user usually
does not look into the log file unless the command executed
returns some hints that something might have gone wrong.
So also return the warnings as an additional return value,
allowing the CLI to report properly.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>