patch 8.2.3651: Vim9: no error for :lock or :unlock with unknown variable

Problem:    Vim9: no error for :lock or :unlock with unknown variable.
Solution:   Give an error. (closes #9188)
diff --git a/src/errors.h b/src/errors.h
index 753e2fe..cde9846 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -686,3 +686,5 @@
 	INIT(= N_("E1244: Bad color string: %s"));
 EXTERN char e_cannot_expand_sfile_in_vim9_function[]
 	INIT(= N_("E1245: Cannot expand <sfile> in a Vim9 function"));
+EXTERN char e_cannot_find_variable_to_unlock_str[]
+	INIT(= N_("E1246: Cannot find variable to (un)lock: %s"));