When stopping with an error, return the item that caused the error

Consider, for example,

```
future.apply::future_lapply(1:10, function(x)
if (x == 8) stop("o no") else x + 1)
```

The error produced is

```
Error in ...future.FUN(...future.X_jj, ...) : o no
```

which doesn't provide any hint that 8 was the guilty input.

I think this is related to but smaller than #32.