gnulib-tool.py: Skip nonexistent modules instead of failing. * pygnulib/GLModuleSystem.py (GLModuleSystem.find): Use the same warning wording as gnulib-tool.sh. * pygnulib/GLImport.py (GLImport.gnulib_cache): Print the specified modules, not the base modules. (GLImport.prepare): Don't put None elements into base_modules.