patch 8.2.3985: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/fold.c b/src/fold.c
index 7d993f7..f8331be 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -563,9 +563,9 @@
     if (foldmethodIsManual(curwin) || foldmethodIsMarker(curwin))
 	return TRUE;
     if (create)
-	emsg(_("E350: Cannot create fold with current 'foldmethod'"));
+	emsg(_(e_cannot_create_fold_with_current_foldmethod));
     else
-	emsg(_("E351: Cannot delete fold with current 'foldmethod'"));
+	emsg(_(e_cannot_delete_fold_with_current_foldmethod));
     return FALSE;
 }