Propagate errors and declare on compare API

Summary

Relates to gitlab-org/gitlab#364176 (closed)

!3 (merged) got stuck, because there was no sensible way to test integration between comparison API and urfave/cli, since the comparison would exit on log.Fatalf. This MR restructures error handling such that errors are propagated to the top level and declared on the API, this allows then to futher propagate the error, which will be handled by urfave/cli library.