length 1 aggregate for all cases and remove warn branch (#7260)

Author: Mukulyadav2004

R/fcast.R

@@ -186,11 +186,7 @@ dcast.data.table = function(data, formula, fun.aggregate = NULL, sep = "_", ...,
     maybe_err = function(list.of.columns) {
       if (!all(lengths(list.of.columns) == 1L)) {
         msg = gettext("Aggregating functions should take a vector as input and return a single value (length=1), but they do not, so the result is undefined. Please fix by modifying your function so that a single value is always returned.")
-        if (is.null(fill)) { # TODO change to always stopf #6329
-          stop(msg, domain=NA, call. = FALSE)
-        } else {
-          warning(msg, domain=NA, call. = FALSE)
-        }
+        stop(msg, domain=NA, call. = FALSE)
       }
       list.of.columns
     }