I love using `plyr`, but sometimes the underlying data throws an error that I can't locate. 

For instance, I've created a summing function that throws an error if `x == 8`:

```
df <- data.frame(x = rep(1:10,3), y = runif(30))

ddply(df,
      .(x),
      function (z) { 
        if(z$x[1] == 8) {
          stop("There's an error somewhere.")
        }
        return(sum(z$y))
        })

```

Pretending I didn't know what had caused the error, is there any way to report which rows of data caused the error?