Propagate COLLADA parsing errors

From #196.

This will likely require some changes in OpenCOLLADA, but ideally a failure to parse should result in an error from COLLADA2GLTF.

This may require calling the validator first before doing conversion.

Start by reviewing #196 and tracing the COLLADA2GLTF conversion path, then determine how OpenCOLLADA parse failures and the validator are exposed. Done means malformed COLLADA input produces an error from COLLADA2GLTF; no file or test path is named in the issue.

Written by the indexing model from the issue text.