Refactor snippet repo path error handling

What does this MR do?

Refs #216273 (closed)

When performing the snippet backfill migration, we check for a specific invalid path error from Gitaly, so that we can handle it appropriately and continue the migration.

This MR refactors that check to be handled within SnippetRepository which feels like a more logical place to handle it (along with the other errors). This also means that when we then come to re-check for that error in other areas, such as the Create/Update services, we won't need to do the same string comparison in multiple places - we can just check for a PathError.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.