Branch indexing fails for outdated pull requests

Problem

Branch indexing aborts when one of the branches or pull requests has an unexpected problem, e.g. when a pull request refers to a deleted branch.

Proposed solutions

• Well-known situations (like the one mentioned above) should be handled gracefully and the corresponding branch jobs should be deleted automatically.
• For unexpected errors, only the current branch should be skipped. Branch indexing should continue with the next branch.