ssh/session-func.c (guile_ssh_blocking_flush): Fix error handling

* ssh/session-func.c (guile_ssh_blocking_flush): Return `error' symbol
  on error.  Assert that the result of libssh procedure is one of the
  valid constants described in `libssh.h'.
  (guile_ssh_connect_x, guile_ssh_authenticate_server): Likewise.
* doc/api-sessions.texi (Sessions): Update.
* NEWS: Update.