Improve the error messages

[korthout]

Is your feature request related to a problem? Please describe.
Most comments by the action for errors are unclear, unhelpful, and appear to come from nowhere.

Example no permission to push to repo:

Git push to origin failed for case5-backport-target with exitcode 1

Describe the solution you'd like
Error messages should be clear about what was attempted, what went wrong, and what can be done to recover. For example:

Backport-action failed to generate a pull request cherry-picking these changes (workflow run). Tried to push branch backport-x-to-y, but not permitted to push to this repo.

You can use a Personal Access Token (PAT) with repo scope as the token input of the actions/checkout-action step to permit pushing to the repo.

Describe alternatives you've considered
..

Additional context
Some other example errors:

Not permitted to create pull request:

Resource not accessible by integration

Unable to cherry-pick

This error is clear about what happened, what went wrong, and how to recover. However, it still appears to come out of nowhere. A link to the workflow run would help.

Backport failed for stable/8.2, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin stable/8.2
git worktree add -d .worktree/backport-16634-to-stable/8.2 origin/stable/8.2
cd .worktree/backport-16634-to-stable/8.2
git switch --create backport-16634-to-stable/8.2
git cherry-pick -x a472372cdb055835682b13515442139655126034 02910366c2d4aff900c41a4dfd8de1f0e926dfae