JXR crashes Maven build with "target/generated-sources/groovy-stubs/main does not exist"

Steps to reproduce:

1. Create a maven project
2. Add gmaven-plugin into build plugins section.
3. Add just Groovy tests into the project. Don't add Groovy sources
4. Execute mvn clean install

Result: The following error happens

Embedded error: Error rendering Maven report: Error while generating the HTML source code of the projet. basedir .../target/generated- sources/groovy-stubs/main does not exist

I would propose making this a warning in the log, not an exception which terminates Maven build.

Proposed patch is attached.