session-func.c (guile_ssh_write_known_host): Change error handling.

* src/session-func.c (guile_ssh_write_known_host): Throw an
  exception on error.  Make return value undefined.
channel-func.c: Change error handling in some procedures.

* src/channel-func.c (guile_ssh_channel_open_session)
  (guile_ssh_channel_request_exec, guile_ssh_channel_request_env)
  (guile_ssh_channel_close): Throw an exception on error.  Make return
  value undefined.
Merge branch 'master' into ssh-server-api
message-func.c, server-func.c: Improve error handling.

* src/message-func.c (guile_ssh_message_reply_default)
  (guile_ssh_message_auth_reply_success)
  (guile_ssh_message_auth_reply_public_key_success)
  (guile_ssh_message_channel_request_reply_success)
  (guile_ssh_message_auth_set_methods_x): Throw an exception on
  error.  Make return value undefined.  Change the description.
* src/server-func.c (guile_ssh_server_set_x)
  (guile_ssh_server_listen_x, guile_ssh_server_handle_key_exchange):
  Likewise.