patch 9.0.1829: Vim9 missing access-checks for private vars

Problem:  Vim9 missing access-checks for private vars
Solution: Use the proper check for private/readonly variable.  Access
          level for a member cannot be changed in a class implementing an
          interface.  Update the code indentation

closes: #12978

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Co-authored-by: Ernie Rael <errael@raelity.com>
diff --git a/src/errors.h b/src/errors.h
index 24977c4..1662043 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3486,6 +3486,8 @@
 	INIT(= N_("E1365: Cannot use a return type with the \"new\" function"));
 EXTERN char e_cannot_access_private_method_str[]
 	INIT(= N_("E1366: Cannot access private method: %s"));
+EXTERN char e_member_str_of_interface_str_has_different_access[]
+	INIT(= N_("E1367: Access level of member \"%s\" of interface \"%s\" is different"));
 
 EXTERN char e_static_cannot_be_followed_by_this[]
 	INIT(= N_("E1368: Static cannot be followed by \"this\" in a member name"));
@@ -3510,4 +3512,4 @@
 EXTERN char e_method_str_type_mismatch_expected_str_but_got_str[]
 	INIT(= N_("E1407: Member \"%s\": type mismatch, expected %s but got %s"));
 
-// E1367, E1371 - E1399 unused
+// E1371 - E1399 unused