[CI bug] If the built docs branch was not deleted, the build job fails but goes green

Summary:

When building docs we create a branch in the form docs-2-29 to create a pull request. If that branch is not deleted and the job is run again, the job will fail in the sense that the created PR will consist of the change in the existing branch and the job would go green.

See https://jenkins.mc-stan.org/blue/organizations/jenkins/BuildDocs/detail/BuildDocs/96/pipeline/8

We can see the following

[](https://jenkins.mc-stan.org/blue/organizations/jenkins/BuildDocs/detail/BuildDocs/96/pipeline/8#step-27-log-21)[](https://jenkins.mc-stan.org/blue/organizations/jenkins/BuildDocs/detail/BuildDocs/96/pipeline/8#step-27-log-22)[](https://jenkins.mc-stan.org/blue/organizations/jenkins/BuildDocs/detail/BuildDocs/96/pipeline/8#step-27-log-23)[](https://jenkins.mc-stan.org/blue/organizations/jenkins/BuildDocs/detail/BuildDocs/96/pipeline/8#step-27-log-24)To https://****:****@github.com/stan-dev/docs.git
 ! [rejected]        docs-2-29 -> docs-2-29 (non-fast-forward)
error: failed to push some refs to 'https://****:****@github.com/stan-dev/docs.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.

This job should probably fail witha descriptive error message.

Start with the BuildDocs Jenkins job and pipeline step 27, using the linked run and its rejected push output as the reproduction path. Re-run the job while the docs-2-29 branch already exists; done means the job reports the non-fast-forward condition as a failure with a descriptive error instead of going green.