Fix import of LFS files in GitHub import

!48512 (merged) may have exposed a bug in GitHub LFS import that caused LFS objects to be downloaded sequentially via LfsImportService rather than in parallel. Instead of returning individual LfsObject objects, the service would import all files and return a Hash indicating success or failure. This error was caught by an exception handler with the message:

undefined method `oid' for [:status, :success]:Array

This caught exception prevented the failure from stalling out the importer process. We may want to reintroduce that behavior.

Relates to #289669 (closed)