exception handling command off

[c-schicho]

the command off throws occasionally exceptions when the user calls the on and off command one after the other within a short period of time, see the screenshot below. when this is happening, the program does not crash, however, the bot is not responding and, thus the user has no information about what's happening.

• improve the off command, change remind_user.stop() to remind_user.cancel()
• catch exceptions thrown in off command.
• provide an error message to the user if an exception is thrown.

#34