Werk #13306: mk_docker.py: Improve error reporting when executed asynchronously

The agent plugin is expected to transport some errors to the Checkmk server to make them visible on the service discovery page. E.g. when the docker python is missing on the target system, this should be displayed to the user.

However, when the plugin is executed in asynchronous mode, the error reporting of the plugin told the agent to drop the error messages in previous versions.

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