Support early stopping too old pre-installed `tidy` command for macOS in the HTML checker
This PR brings early stopping the HTML checker before errors, which leave some macOS users confused, and suggesting installing a newer `tidy` command.
The pre-installed `tidy` command on macOS is too old, released on 31 October 2006. Additionally, I can see the same date at [StackOverflow](https://stackoverflow.com/questions/22283382/overwrite-osx-tidy
) seven years ago. The `tidy` does not support two indispensable options: `--mute-id` and `--mute`. So, the `./x.py test` command fails with a bunch of errors due not to muting them.
I could confirm the `./x.py test` command before installing a newer `tidy` failed and its command after the installation succeeded.
f2f55e8da9