patch 8.2.2777: Vim9: blob operations not tested in all ways

Problem:    Vim9: blob operations not tested in all ways.
Solution:   Run tests with CheckLegacyAndVim9Success().  Make blob assign with
            index work.
diff --git a/src/errors.h b/src/errors.h
index 99f3aaf..1e09186 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -403,3 +403,5 @@
 	INIT(= N_("E1182: Blob required"));
 EXTERN char e_cannot_use_range_with_assignment_operator_str[]
 	INIT(= N_("E1183: Cannot use a range with an assignment operator: %s"));
+EXTERN char e_blob_not_set[]
+	INIT(= N_("E1184: Blob not set"));