Fix: Conversion failure

[mikhail-dcl]

Fixes https://github.com/decentraland/unity-explorer/issues/1291

• Introduced 5% tolerance for failing conversion
• Introduced downloading in batches to prevent SSL error
• Introduced persistent settings to save parameters of the conversion window into Editor Prefs
• Introduced partial downloading till completion instead of full clean-up, useful for debugging/testing with a lot of required assets for the scene