don't leave partial forward ports #1307
Description
Reported by yoni on odoo/odoo#244822: the batch has two PRs, the first PR ported fine but the second PR failed due to https://www.githubstatus.com/incidents/cqb5hcy0gx18
This caused the forward-port to be retried a bit later, which was not an issue, however at that point the FW was blocked because the odoo/odoo branch was already present, making the push of the community forward port failed. Deleting the existing branch was sufficient to fix the issue.
- if the forward port of a PR fails, try to delete any branch previously created for other PRs in the batch
- if that does not succeed, trigger a notification (not just a mail.thread message), as the error can not resolve itself (future attempts to forward port will be divergent)
- technically we might want to check trees and consider the port a success, but doing that atomically seems iffy
- we could also force-push the branch, but that can trigger data loss which seems undesirable