LIBSSH2_ERROR_ALLOC - An internal memory allocation call failed.
LIBSSH2_ERROR_SOCKET_SEND - Unable to send data on socket.
LIBSSH2_ERROR_SFTP_PROTOCOL - An invalid SFTP protocol response
was received on the socket, or an SFTP operation caused an errorcode
to be returned by the server. In particular, this can be returned if
the SSH server does not support the fsync operation: the SFTP subcode
LIBSSH2_FX_OP_UNSUPPORTED will be returned in this case.