cmd-coreos-prune: handle expired or deleted tags in skopeo inspect

When a tag is expired or been garbage collected in Quay, skopeo inspect
returns a "unknown" error with a message like:

"Tag <tag> was deleted or has expired. To pull, revive via time machine"

Check for this condition and skip raising the exception, instead just
return since the tag has already been GC'd.

Author: marmijo

src/cmd-coreos-prune

@@ -520,6 +520,12 @@ def skopeo_inspect(repo, tag, auth):
         if exit_code == 2:
             print(f"\t\t\tSkipping deletion for {repo}:{tag} since the tag does not exist.")
             return False
+        # Quay could return an "unknown" error code when a tag has expired or been GC’d.
+        # Check for this error message specifically and avoid raising an exception in that
+        # case, since the image is already gone.
+        elif "Tag" in error_message and "was deleted or has expired" in error_message:
+            print(f"\t\t\tSkipping deletion for {repo}:{tag} since the tag was deleted or has expired.")
+            return False
         else:
             # Handle other types of errors
             raise Exception(f"Inspection failed for {repo}:{tag} with exit code {exit_code}: {error_message}")