fix(gc): graceful stop to offline gc if root repositories path non-exist

Related to #742 (closed)

🌱 Context

See #742 (closed) for full context on why we are making this change.

In summary, we want to be able to exit gc early without failing when the root directory that houses the repositories in the registry in non existent. The current behaviour as detailed in #742 (closed) causes the gc process to be marked as failed when the directory is non existent. To address this we have:

• Added a check for the root directory existence at the top of the mark and sweep stage
• Asserted that the error returned by the check mentioned above will trigger gc to be skipped or not; following the proposed implementation