patch 8.2.3284: no error for insert() or remove() changing a locked blob

Problem:    No error for insert() or remove() changing a locked blob.
Solution:   Check a blob is not locked before changing it. (Sean Dewar,
            closes #8696)
diff --git a/src/eval.c b/src/eval.c
index 3688176..550fe8e 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1749,7 +1749,7 @@
 	    }
 	    else
 	    {
-		emsg(_(e_listreq));
+		emsg(_(e_string_list_or_blob_required));
 		clear_tv(&tv);
 	    }
 	}