[commands] Fix Context.command_failed from being incorrect.

When used, it would be set to False after the invoke was done. Ideally
it should report to False during invoke but True during any error
case.

discord/ext/commands/context.py

@@ -83,7 +83,7 @@ class Context(discord.abc.Messageable):
         self.invoked_with = attrs.pop('invoked_with', None)
         self.invoked_subcommand = attrs.pop('invoked_subcommand', None)
         self.subcommand_passed = attrs.pop('subcommand_passed', None)
-        self.command_failed = attrs.pop('command_failed', True)
+        self.command_failed = attrs.pop('command_failed', False)
         self._state = self.message._state
 
     @asyncio.coroutine