[JENKINS-73791] Prevent PR 404 on details to break Branches scan

[Dohbedoh]

Description

JENKINS-73791: The retrieval of branches can fail the entire Branch Scanning if a BranchDiscoveryTrait strategy that involves checking on PRs is used while some PR parts cannot be retrieved with a 404 FileNotFoundException.

This is an extension of JENKINS-56996 / #224.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Automated tests have been added to exercise the changes
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verify that the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify

@jenkinsci/github-branch-source-plugin-developers

[Dohbedoh]

First commit to reproduce the failure.
Second commit coming soon to demonstrate the fix.

[Dohbedoh]

@dwnusbaum @jeromepochat @rsandell Can we merge this ?