Show staging failure history in PRs / failure message

[xmo-odoo]

Should be more feasible with the v2 refactoring as it shoudl be much easier to link a PR to its successive stagings. Currently the final failure is linked, however when there's an ambient background of non-deterministic issues, it's possible for a PR to have an error breaking the first 4 stagings, then on the very last staging (which will trigger the terminal failure notification) it hits a non-deterministic error, and the user looks at the build and goes "doesn't seem relevant", and immediately retries, blowing up 5 new stagings and creating hours more integration delay.