A responder with all exit codes zero prints errors and returns them. The caller uses cli.IsValidationError(err) to choose its own program exit codes, after cleanup.
Run from the repository root:
mise example error-handling "Hello, World!"
# Hello, World!
# program exit code: 0
mise example error-handling --unknown
# Error: unknown flag --unknown
# <usage text>
# program exit code: 64
mise example error-handling --fail
# Error: simulated execution failure
# program exit code: 70