Some Scene assets are not converted to Asset Bundles

Every now and then we see some scenes exiting with error code 4, meaning that not all of its assets were converted, we should debug those cases and see if anything can be done to fix those issues.

All the relevant info and instructions is at the [ABs Playbook](https://playbooks.decentraland.systems/world-team/asset-bundles.html), and the scene conversion logs can be seen at https://unityab.decentraland.io/#/logentry

From what I could see (for example in [this case](https://unityab.decentraland.io/#/logentry/397541/show) with [this log](https://dev-assetbundles-8af4af4.s3.amazonaws.com/logs//opt/unity/Editor/export_log_QmPiUjxMsC2xnn87GY4nUubqeYj2spSCHex48RMwriKCJe_1620729512.txt)) it's normally a problem with some materials name, and we've seen in the past this errores occur when a "broken" GLTF is used in the scene, for example a GLTF that points to an external texture that hasn't been deployed with the scene. Of course those cases can't be fixed by us.