R maintenance Feb2017 (#2045)

* [R] better argument check in xgb.DMatrix; fixes #1480

* [R] showsd was a dummy; fixes #2044

* [R] better categorical encoding explanation in vignette; fixes #1989

* [R] new roxygen version docs update

R-package/R/xgb.DMatrix.R

@@ -20,6 +20,9 @@
 xgb.DMatrix <- function(data, info = list(), missing = NA, ...) {
   cnames <- NULL
   if (typeof(data) == "character") {
+    if (length(data) > 1)
+      stop("'data' has class 'character' and length ", length(data),
+           ".\n  'data' accepts either a numeric matrix or a single filename.")
     handle <- .Call("XGDMatrixCreateFromFile_R", data, as.integer(FALSE),
                     PACKAGE = "xgboost")
   } else if (is.matrix(data)) {