When `bin/qunit` is given a path that doesn't exist (e.g. missing `.js`
extension or wrong directory), it previously printed a warning but
continued without any file filter. This caused the entire test suite to
run by accident instead of just the intended file.
This changes the warning to an error with `exit 1` so the script fails
fast, avoiding accidentally running all tests.
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Co-authored-by: Jarek Radosz <jarek@cvx.dev>