Related docs error messages can be improved

Some error messages logged by the related docs plugins are pretty cryptic, and should be made more clear.

For example, I recently forgot to add the property 'model.compareTo' to a template node for the related docs plugin in a document type. The related docs plugin then logs the following error when closing a document that has been edited before:

"no base model service configure"

That's neither correct english nor very helpful to figure out what's wrong. The only way to discover the error was checking out the source code and looking up the log message in there... This particular message could easily be changed to something like:

"missing property 'model.compareTo' in related documents plugin configuration, falling back to view mode instead of compare mode"

The same holds for the log message "no base model service available".