Migrate from golang.org/x/errors
In https://gitlab.com/gitlab-org/ci-cd/custom-executors/autoscaler/merge_requests/10 we started using the new error handling in Go that is going to be introduced in Go 1.13, but currently, we are running Go 1.12.
When Go 1.13 is out we should remove all golang.org/x/errors
and use the stdlib error package.