Provide better feedback when projects fail to resolve

Currently, if you declare projectScope mappings for project paths that don't exist, there is no error logged.  The resulting module will have incorrect module.xml that is missing the 'scope' value for the `<jar scope=""></jar>` elements.  Without this, the module will fail to run.

We should validate the project paths exist as part of the plugin, and throw or log an error if they aren't found.