ssh/auth.c: Throw `wrong-type-arg' instead of `guile-ssh-error'

* ssh/auth.c: Throw `wrong-type-arg' instead of `guile-ssh-error' if a
  disconnected session is passed as an argument to procedures in the
  module.
* ssh/session-type.h (GSSH_VALIDATE_CONNECTED_SESSION): New macro.
* doc/api-auth.texi (Auth): Update.