exit20 for usage of unavailable package_set

src/mpsd_software_manager/mpsd_software.py

@@ -919,7 +919,7 @@ def install_environment(
             msg += f" in release {mpsd_release}. "
             msg += "Use 'available' command to see list of available package_sets."
             logging.error(msg)
-            sys.exit(1)
+            sys.exit(20)
 
     # Install the package_sets
     with os_chdir(package_set_dir):