Need better error message when fetch_assay_data() is called with invalid features

This call:

```r
library(FacileData)
fds <- exampleFacileDataSet()
fetch_assay_data(fds, "mygene")
```

Produces this error message

```
Error in melt.data.table(vals, id.vars = "rn", variable.factor = FALSE,  : 
  One or more values in 'id.vars' is invalid.
```

Which leaves the user to wonder what the heck just happened.
