Better error message when on= columns wrong way around

[mattdowle]

require(data.table)
X = data.table(a=1:3,b=4:6,c=c("foo","bar","baz"))
Y = data.table(A=2:4, B=5:7)
X[Y, on=c(A="a")]
# Error in forderv(x, by = rightcols) : 
#   'by' value -2147483648 out of range [1,3]
X[Y, on=c(a="A")]
#    a  b   c B
#1: 2  5 bar 5
#2: 3  6 baz 6
#3: 4 NA  NA 7