have better error message for a case of missing/misnamed variable

[paciorek]

Here's a case where we should explicitly say that y is not found.

go = nFunction(
   fun = function(x = 'numericVector') {
       for(i in 1:length(y)) {}
       })
cgo <- nCompile(go)

An error occurred in compiler stage labelAbstractTypes (for method or nFunction nFun_10_NFID_14):
argument is of length zero
This occurred while: handling labelAbstractTypes for y.
Error in if (code$args[[1]]$type$nDim >= 2) { : 
  argument is of length zero

I will note that in other scenarios we do better messaging, so there's probably something about the nesting that needs to be investigated.

go = nFunction(
   fun = function(x = 'numericVector') {
       y <- z
       })
cgo <- nCompile(go)

An error occurred in compiler stage labelAbstractTypes (for method or nFunction nFun_9_NFID_13):
variable 'z' has not been created yet.
This occurred while: handling labelAbstractTypes for z.
Error: variable 'z' has not been created yet.