Add more graceful response if successful challenge-response but the challenger quit the session

[therealpaulgg]

Example of this behavior:

Client starts challenge, and quits (ctrl+c)

On other machine this happens:

ssh-sync challenge-response
Please enter the challenge phrase: stranger-enzyme-buckshot
failed to parse PEM encoded key: failed to decode PEM data

Expected behavior would be that the challenge should clean itself up when the challenger quits their session, and when the challenge response occurs it should say invalid challenge.

ssh-sync challenge-response
Please enter the challenge phrase: stranger-enzyme-buckshot
error from server: Invalid challenge response

[therealpaulgg]

This is fixed in this pull request. therealpaulgg/ssh-sync-server#17