patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/fold.c b/src/fold.c
index 75b436a..f10294d 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -1762,7 +1762,7 @@
 {
     if (!curbuf->b_p_ma)
     {
-	emsg(_(e_modifiable));
+	emsg(_(e_cannot_make_changes_modifiable_is_off));
 	return;
     }
     parseMarker(curwin);