ogcli: improve handling of subcommand errors

If no subcommand or an invalid one is specified print an error message
followed by ogcli --help. Also exit with status code 1.

ogcli

@@ -42,9 +42,15 @@ class CLI():
                                      if not attr.startswith('_')])
         args = parser.parse_args(sys.argv[1:2])
 
+        if args.command is None:
+            print('Missing subcommand', file=sys.stderr)
+            parser.print_help(file=sys.stderr)
+            sys.exit(1)
+
         if not hasattr(self.ogcli, args.command):
-            parser.print_help()
-            sys.exit('Unknown command')
+            print('Invalid subcommand', file=sys.stderr)
+            parser.print_help(file=sys.stderr)
+            sys.exit(1)
 
         # Call the command with the same name.
         getattr(self.ogcli, args.command)(sys.argv[2:])