message: Add a better error message than `syscall 318 not found`
diff: @@ -126,7 +126,13 @@ impl<'a, 'tcx> EvalContextExt<'tcx> for EvalContext<'a, 'tcx, super::Evaluator>
  }
  "syscall" => {
  + // TODO: read `syscall` ids like `sysconf` ids and
  + // figure out some way to actually process some of them
  + //
  + // libc::syscall(NR_GETRANDOM, buf.as_mut_ptr(), buf.len(), GRND_NONBLOCK)
  + // is called if a `HashMap` is created the regular way.
  match self.value_to_primval(args[0], usize)?.to_u64()? {
  + 318 |
  511 => return Err(EvalError::Unimplemented("miri does not support random number generators".to_owned())),
  id => return Err(EvalError::Unimplemented(format!("miri does not support syscall id {}", id))),
  }
  