/packit propose-downstream comment in a issue retriggers also non failed targets (and fails)

[majamassarini]

When retriggering propose-downstream from an issue, packit-service runs the propose-downstream on all the configured targets and not just for the one reported as failed.

On the non-failed targets the PRs were already merged and this caused packit-service to report an error for the already released targets.

We can:

• re-run the propose-downstream only for the failed target when the retriggering comes from the packit-service generated issue.
• handle this exception as a warning instead of an error and in general mark the task as skipped instead of failed (in this way we should avoid to report an error to the user).