Add a way to show warning/error messages back to the user

[Marc Cluet]

One feature that we're missing from juju on our deployments is the capability to be able to send a message back to the user when doing the deployments.

Right now this is not possible and the only way to remotely do this is by going to the node and tailing the logs there, which is not ideal in big installations.

[Dustin Kirkland  (kirkland)]

An example here...

Upon destroying the swift service, we need to inform the user that all data on this system will be lost, AND 1TB of data will need to be resync'd around the network to the other swift nodes in the cluster.

[Clint Byrum (clint-fewbar)]

I think the prescribed mechanics are to error out some hook which will cancel the destruction of the service. I don't know if an error in the 'stop' hook would do this, but its currently the only one that runs before the service unit is taken down.

One can actually filter the debug-log, which might work.

juju debug-log --include 'servicename*'

Because of the async nature, whats probably needed is the ability to block until a state change that you would expect. So

juju destroy-service swift --wait-and-log

Which would wait for the state transition and show the debug-log for said service while waiting. This would actually make juju feel very interactive.