Improve error handling and general cleanup

[jesperes]

General cleanup PR:

• Better error message when there are no commits in the repo
• Run Dialyzer with -Wunknown
• Run Dialyzer in GitHub workflows
• Fix a bunch of Dialyzer errors
• Removed some unused code

[jesperes]

This PR has been tested using Klarna's internal CI, and is ready for merging.