Improve final backporting result reports

**Is your feature request related to a problem? Please describe.**
Right now if something went wrong, we list all failures that occurred without linking it to the specific target branch to which it is associated with. This could be annoying when there are many issues or long errors.

**Describe the solution you'd like**
As final step, returns a report such that it clearly states which backport failed and which did not.

For instance, something in tabular format:

| Target Branch | Result | Error |
|---------------|--------|-------------|
| develop | OK | - |
| prod | KO | error: .... |

**Describe alternatives you've considered**
n/a

**Additional context**
This could also be added in the failure comment added in the original pull request

The issue names no files, tests, or entry points. Start by locating the final backport result aggregation and the failure comment path; completion means producing one target-branch/result row for each backport, including failures and their errors.

Written by the indexing model from the issue text.