Better clarification of error messages in `pmm-admin` when PMM server can't be unregistered

There are some cases where pmm-admin error message doesn't make much sense. For example here the error is not due to incorrect username/password but because the operation is forbidden by PMM Server so the error should be more generic.

[ec2-user@ip-10-178-2-29 ~]$ pmm-admin unregister --force --node-name=pmm-server
PMM Server node can't be removed.. Please check username and password.

The first half of the error message is from pmm-managed, the second part is from pmm-admin in response to the 403 by pmm-managed.

https://github.com/percona/pmm-admin/blob/master/main.go#L179-L197

More info

https://perconacorp.slack.com/archives/G2HEW592P/p1623398121177900