gh-104683: Rework Argument Clinic error handling (#107551)

Introduce ClinicError, and use it in fail(). The CLI runs main(),
catches ClinicError, formats the error message, prints to stderr
and exits with an error.

As a side effect, this refactor greatly improves the accuracy of
reported line numbers in case of error.

Also, adapt the test suite to work with ClinicError.

Co-authored-by: Alex Waygood <[email protected]>

Author: erlend-aasland