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/indent.c b/src/indent.c
index e1c6f52..a26344b 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -991,7 +991,7 @@
     // Don't even try when 'modifiable' is off.
     if (!curbuf->b_p_ma)
     {
-	emsg(_(e_modifiable));
+	emsg(_(e_cannot_make_changes_modifiable_is_off));
 	return;
     }