Add SftpException and SftpPathNotFoundException.Path #1716
                
     Merged
            
            
          
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This adds an SftpException which sits between the existing SftpPathNotFoundException/SftpPermissionDeniedException and SshException, and which contains the response code from the SSH_FXP_STATUS packet, along with a default message if one was not provided.
SftpPathNotFoundException also gains a Path property which is populated in cases where it makes sense.
closes #161
closes #567