MockModuleBuilder: Don't run the final createrepo if the module build failed
    
    The build steps assume that the the repository for the build is non-modular,
    so after the final createrepo is run with include_module_yaml=True, it's
    impossible to repeat a build steps. Running mock -c <failed_config> build is
    quite useful for debugging a failed build (even though it's hard to find the
    right config) - skipping the final createrepo step for failed module builds
    makes it work as expected.