patch 8.2.3720: Vim9: Internal error when invoking closure in legacy context

Problem:    Vim9: Internal error when invoking closure in legacy context.
Solution:   Give a more appropriate error message. (closes #9251)
diff --git a/src/errors.h b/src/errors.h
index e09ef53..1d6a3a3 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -689,3 +689,5 @@
 	INIT(= N_("E1246: Cannot find variable to (un)lock: %s"));
 EXTERN char e_line_number_out_of_range[]
 	INIT(= N_("E1247: Line number out of range"));
+EXTERN char e_closure_called_from_invalid_context[]
+	INIT(= N_("E1248: Closure called from invalid context"));