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): Likewise.