usage check if command exist before request usage

cccli/command/shell.py

@@ -87,6 +87,8 @@ class Command_usage(Command):
     def __call__(self, argv):
         if len(argv) != 2:
             raise cmdBadArgument()
+        if argv[1] not in self.cli.commands:
+            raise cmdBadArgument(argv[1])
         usage = self.cli.commands.usage(argv[1])
         if usage is None:
             self.printer.out("No usage.")