feat(rustdoc): harmonise error messages
Based on unix tools wording, it follows a standard format: `program_name: context: error message`, potentially prompting the user to use the `--help` option.
This is clearly meant to trigger some discussion on #38084, as messages still use `stdout` and `stderr` somewhat arbitrarily, and there are a few `error!()` calls as well.
b1a2ab86db