Improve context in mock error message

[jjtt]

Trying to mock the db for a codebase that uses multiple inserters and it's a bit of a learning curve to figure out what needs to be done.

One small change that made things easier for me was just adding req to the panic message here:

https://github.com/loyd/clickhouse.rs/blob/cef34f73e71d4cd25446475390e9b3b3803f0a40/src/test/mock.rs#L62