21429 - "mco plugin doc" fails with a unclear message when trying to describe a plugin that does not exist

In the past using mco plugig doc to describe a plugin that doesn't
exist but the type exists failed by raising an undescriptive exception.
An example of this would be mco plugin doc agent/rpcut

This commit changes the exception to a descriptive error message in the
style of "Could not find a 'agent' plugin named 'rpcut'"