Improve logging in FolderTreePlugin when misconfigured

Let's give more information in the warning "The configured path '"startingPath"' does not exist, using /content instead." in the constructor org.hippoecm.frontend.plugins.cms.browse.tree.FolderTreePlugin

As a developer I'd like to know what plugin instance this concerns, where the configuration is and what I can do to fix it (create this content folder in the project)