patch 9.0.1982: vim9: clean up from v9.0.1955

Problem:  vim9: clean up from v9.0.1955
Solution: Fix a few remaining issues, improve error message

- Use `cl_exec`, the executing class, to check permissions in `get_lval()`.
- Handle lockvar of script variable from class.
- Add 'in class "Xxx"' to e_cannot_access_private_variable_str.

closes: #13222

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>
diff --git a/src/vim9class.c b/src/vim9class.c
index 05d9afc..ce23160 100644
--- a/src/vim9class.c
+++ b/src/vim9class.c
@@ -2172,7 +2172,8 @@
 
     if (*name == '_')
     {
-	semsg(_(e_cannot_access_private_variable_str), m->ocm_name);
+	semsg(_(e_cannot_access_private_variable_str), m->ocm_name,
+							    cl->class_name);
 	return FAIL;
     }
 
@@ -2331,7 +2332,8 @@
 
 	if (*name == '_')
 	{
-	    semsg(_(e_cannot_access_private_variable_str), m->ocm_name);
+	    semsg(_(e_cannot_access_private_variable_str), m->ocm_name,
+							    cl->class_name);
 	    return FAIL;
 	}