ssh/error.c (guile_ssh_session_error1): New procedure

* ssh/error.c (guile_ssh_session_error1): New procedure.
* ssh/error.h: Likewise.
* ssh/channel-func.c (guile_ssh_channel_open_session)
  (guile_ssh_channel_request_exec, guile_ssh_channel_request_pty)
  (guile_ssh_channel_request_shell, guile_ssh_channel_request_env): Use it.
* ssh/channel-type.c (ptob_write, ptob_flush): Use it.
* ssh/server-func.c (guile_ssh_server_accept)
  (guile_ssh_server_handle_key_exchange): Use it.
* ssh/session-func.c (guile_ssh_connect_x)
  (guile_ssh_authenticate_server, guile_ssh_write_known_host): Use it.