message: Improve deterministic mode error message
diff: @@ -795,8 +795,10 @@ fn gen_random<'a, 'mir, 'tcx>(this: &mut MiriEvalContext<'a, 'mir, 'tcx>,
  }
  None => {
  err!(Unimplemented(
  - "miri does not support random number generators in deterministic mode!
  - Use '-Zmiri-seed=<seed>' to enable random number generation".to_owned(),
  + "miri does not support gathering system entropy in deterministic mode!
  + Use '-Zmiri-seed=<seed>' to enable random number generation.
  + WARNING: Miri does *not* generate cryptographically secure entropy -
  + do not use Miri to run any program that need secure random number generation".to_owned(),
  ))
  }
  }
  