Clean up output in response to C-c

[exarkun]

C-c is the only way to exit kubetop right now. It can leave unsightly blemishes on the terminal:

^CTraceback (most recent call last):
Failure: twisted.internet.defer.FirstError: FirstError[#1, [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.defer.FirstError'>: FirstError[#1, [Failure instance: Traceback (failure with no frames): <class 'twisted.web._newclient.ResponseFailed'>: [<twisted.python.failure.Failure twisted.internet.error.ConnectionLost: Connection to the other side was lost in a non-clean fashion: Connection lost.>, <twisted.python.failure.Failure twisted.web.http._DataLoss: Chunked decoder in 'BODY' state, still expecting more data to get to 'FINISHED' state.>]
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