patch 8.2.1867: Vim9: argument to add() not checked for blob

Problem:    Vim9: argument to add() not checked for blob.
Solution:   Add the BLOBAPPEND instruction.
diff --git a/src/errors.h b/src/errors.h
index 8ddf441..4d8456f 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -284,4 +284,6 @@
 	INIT(= N_("E1129: Throw with empty string"));
 EXTERN char e_cannot_add_to_null_list[]
 	INIT(= N_("E1130: Cannot add to null list"));
+EXTERN char e_cannot_add_to_null_blob[]
+	INIT(= N_("E1131: Cannot add to null blob"));
 #endif