Just say "invalid" instead of "missing"

Author: MichaelChirico

src/assign.c

@@ -119,7 +119,7 @@ static int _selfrefok(SEXP x, Rboolean checkNames, Rboolean verbose) {
   }
   p = R_ExternalPtrAddr(v);
   if (p==NULL) {
-    if (verbose) Rprintf(_("This table is missing data.table internal attributes. This is expected and normal for a data.table loaded from disk. Please remember to always setDT() immediately after loading to prevent unexpected behavior. If this table was not loaded from disk or you've already run setDT(), please report to the data.table issue tracker.\n"));
+    if (verbose) Rprintf(_("The data.table internal attributes of this table are invalid. This is expected and normal for a data.table loaded from disk. Please remember to always setDT() immediately after loading to prevent unexpected behavior. If this table was not loaded from disk or you've already run setDT(), please report to the data.table issue tracker.\n"));
     return -1;
   }
   if (!isNull(p)) internal_error(__func__, ".internal.selfref ptr is neither NULL nor R_NilValue"); // # nocov