Make clear that a missing configurator is normally a plugin that hasn't loaded or is missing

[timja]

Describe your use-case which is not covered by existing documentation.

See discussion around: https://gitter.im/jenkinsci/configuration-as-code-plugin?at=61fa665aced11857f98d53c6

The error message confused the user, we could do better I think, by saying to check plugin is installed, and to check the log and make sure the plugin didn't fail to load

https://gitter.ems.host/_matrix/media/v1/download/systemausfall.org/XdRIDoidqzBALOsKszsHRAce

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

No response