A nicer error if the value column doesn't exist

Hi,

I had a typo in the name of the value column and it led to the not especially enlightening error

``` R
Error in dim(ordered) <- c(attr(row_id, "n"), attr(col_id, "n")) : 
  attempt to set an attribute on NULL
```

It would be nice if you could fit in a stopifnot (or assertthat) somewhere along the way. :)

Thx
Stefan

Version: Current CRAN on R 3.1.1
