Improve error message when `release-note-appender` failed to create PR

[kavirajk]

When release-note-appender action fails to create automatic PR, it shows error which is not that helpful.

Example:
grafana/loki#10281 (comment)

The process '/usr/bin/git' failed with exit code 1

Would be great to have more details on why it failed.

Context

This error was faced when add-to-release-notes label was removed and added second time (first GH action failed due to rate limits)