further improvements to error reporting

git-svn-id: https://svn.r-project.org/R/trunk@87789 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

src/library/base/R/unlist.R

@@ -19,7 +19,8 @@
 unlist <- function(x, recursive=TRUE, use.names=TRUE)
 {
     ## for better error messages (islistfactor is not viisble)
-    if(length(recursive) > 1L) stop("'recursive' must be of length 1")
+    if(length(recursive) != 1L) stop("'recursive' must be of length 1")
+    if(is.na(recursive)) stop("'recursive' is NA")
     if(.Internal(islistfactor(x, recursive))) {
         URapply <-
             if(recursive) # use rapply()