[Rd] Misinformative error message for vector()

[Henrik Bengtsson]

Nothing serious, but I just noticed that if you pass a function as the
'length' argument of vector(), you get the following misinformative
error message:

> vector("list", length=function() {})
Error in vector("list", length = function() { :
  negative length vectors are not allowed

whereas if you for instance do:

> vector("list", length="foo")
Error in vector("list", length = "foo") :
  unimplemented type 'character' in 'asVecSize'

which is more informative.

/Henrik