Add command exit code to output if it fails

[KaindlJulian]

Implements the first point from #341. The exit code is included in the error message that is already being printed.

e.g.

$ hyperfine false
Benchmark #1: false
Error: Command terminated with non-zero exit code: 1. Use the '-i'/'--ignore-failure' option if you want to ignore this. Alternatively, use the '--show-output' option to debug what went wrong.",

[sharkdp]

Thank you very much for your contribution!

Could you please add an entry to the "unreleased" section in CHANGELOG.md? The format for entries is:

- Description what has been changed, see #123 (@user)

where #123 links to the bug ticket and/or the PR and user is your username.

[sharkdp]

Thank you for the update!

[sharkdp]

Released in https://github.com/sharkdp/hyperfine/releases/tag/v1.12.0