Provide return blocked operation message to a user.

[anastasiamac]

Previously, when juju model was locked for any type of change - destruction, removal, changes to Juju components such as models, machines, units, etc - when a user ran any of the commands that are blocked, the command will return silently without providing any feedback. There was no way for user to know that the command was blocked except to examine the log.

With this PR, we now inform the user that the operation was blocked and that the environment is locked.

Adjust unit tests accordingly.

• QA steps *
• Bootstrap
• Run "disable-command" to block operations
• Run any of the disabled commands
  [observe command feedback that operation is blocked]

[anastasiamac]

@dimitern \o/ tyvm!
$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju