Refactored duplicated logic in setDT and [,:=] to use a helper function

Author: nipun-gupta-3108

inst/tests/tests.Rraw

@@ -3295,7 +3295,7 @@ test(1034, as.data.table(x<-as.character(sample(letters, 5))), data.table(V1=x))
   test(1035.291, melt(dt, measure.vars=NA_integer_, id.vars=NULL), error="One or more values in 'measure.vars'")
   test(1035.30, melt(dt, id.vars=NA_integer_), error="One or more values in 'id.vars'")
   test(1035.31, melt(dt, measure.vars=NA_character_), error="One or more values in 'measure.vars'")
-  test(1035.32, melt(dt, id.vars=NA_character_), error="One or more values in 'id.vars' is invalid.")
+  test(1035.32, melt(dt, id.vars=NA_character_), error="One or more values in 'id.vars'.")
 
   if (test_R.utils) {
     # dup names in variable used to generate malformed factor error and/or segfault, #1754;  was test 1570