pkg-merge-upstream-to-downstreams: Explain failed merges

Tell users what to do when automatic merges fail and they need to fix conflicts manually by printing a summary like this at the end of the merge-updates pipeline:

⚠️  Failed to automatically merge the updates below.

Check out each proposed branch, manually merge it to the downstream_branch and push it to update the conflicting merge requests:

  Failed to merge debian/bullseye into apertis/v2022dev3 via proposed-updates/debian/bullseye/56f5c7b1:
  git pull && git checkout proposed-updates/debian/bullseye/56f5c7b1 && git merge origin/apertis/v2022dev3 && git push origin proposed-updates/debian/bullseye/56f5c7b1