[micuele]

This fixes several issues in the game importer:

• Failed imports now show the actual error and allow another attempt.
• Downloads finish writing before extraction, and stalled transfers time out.
• Artifacts are extracted and checked before replacing the installed game, so extraction or quota failures preserve the existing version.
• Disk quota checks account for the extracted game size and the version being replaced.
• Temporary files are cleaned up after failed imports.
• Existing Lean toolchains are reused.

This still needs to be tested on the dev instance of lean4game.