message: correct factual mistake in Windows message, and use "+" sugar
diff: @@ -100,11 +100,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let NANOS_PER_INTERVAL = NANOS_PER_SEC / INTERVALS_PER_SEC;
  let SECONDS_TO_UNIX_EPOCH = INTERVALS_TO_UNIX_EPOCH / INTERVALS_PER_SEC;
  - let duration = system_time_to_duration(&SystemTime::now())?
  - .checked_add(Duration::from_secs(SECONDS_TO_UNIX_EPOCH))
  - .unwrap();
  + let duration = system_time_to_duration(&SystemTime::now())? + Duration::from_secs(SECONDS_TO_UNIX_EPOCH);
  let duration_ticks = u64::try_from(duration.as_nanos() / u128::from(NANOS_PER_INTERVAL))
  - .map_err(|_| err_unsup_format!("programs running longer than 2^64 ticks are not supported"))?;
  + .map_err(|_| err_unsup_format!("programs running more than 2^64 Windows ticks after the Windows epoch are not supported"))?;
  let dwLowDateTime = u32::try_from(duration_ticks & 0x00000000FFFFFFFF).unwrap();
  let dwHighDateTime = u32::try_from((duration_ticks & 0xFFFFFFFF00000000) >> 32).unwrap();
  