patch 8.2.4408: Vim9: some code not covered by tests

Problem:    Vim9: some code not covered by tests.
Solution:   Add a few more tests.  Correct error message.  Allow unlet on dict
            with a number key.
diff --git a/src/errors.h b/src/errors.h
index 8e2b9da..5f2685a 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3010,8 +3010,7 @@
 	INIT(= N_("E1180: Variable arguments type must be a list: %s"));
 EXTERN char e_cannot_use_underscore_here[]
 	INIT(= N_("E1181: Cannot use an underscore here"));
-EXTERN char e_blob_required[]
-	INIT(= N_("E1182: Blob required"));
+// E1182 unused
 EXTERN char e_cannot_use_range_with_assignment_operator_str[]
 	INIT(= N_("E1183: Cannot use a range with an assignment operator: %s"));
 #endif