COLLADA2glTF Silently Fails on Zipped Models

Replaces https://github.com/KhronosGroup/glTF/issues/673 by @KermMartian

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>Although COLLADA2glTF links against libz, making me assume it can understand model + textures zipped into a compressed archive, it generates a 474-byte gltf file containing no geometry. For example:
`baz@bar$ .foo/collada2gltf -r -e -f /somedir/struct.dae.zip -o /somedir/struct.gltf
converting:/somedir/struct.dae.zip ... as /somedir/struct.gltf
[Info]: current working directory:/someotherdir
[geometry] 0 bytes
[animations] 0 bytes
[scene] total bytes:0
[completed conversion]
Runtime: 0.00 seconds`
>
>It should either properly convert the model, or explicitly emit an error.

Run the documented collada2gltf command with the zipped model and confirm whether it produces an empty glTF without an error. Trace how the .dae.zip input is handled, then make the completed behavior either a valid conversion or an explicit error instead of a geometry-free success; verify both outcomes with a representative zipped model.

Written by the indexing model from the issue text.