Bug 1179488 - Accept any arguments passed to deprecated commands; r=gps

Without this, we throw UnrecognizedArgumentError when running commands
such as `./mach mochitest-plain test`, which causes an error message
such as the below to be emitted:

It looks like you passed an unrecognized argument into mach.

The mochitest-plain command does not accept the arguments: test

This patch will fix the above command to instead print the corresponding
deprecation message.

python/mach/mach/dispatcher.py

@@ -230,7 +230,7 @@ class CommandAction(argparse.Action):
                 setattr(command_namespace, name, extra)
             else:
                 setattr(command_namespace, name, options.get('default', []))
-        elif extra:
+        elif extra and handler.cls.__name__ != 'DeprecatedCommands':
             raise UnrecognizedArgumentError(command, extra)
 
     def _handle_main_help(self, parser, verbose):