Asset loading hangs forever when errors hit loading an asset.

[diarmidmackenzie]

Description:

• A-Frame Version: 1.3.0
• Platform / Device: All
• Reproducible Code Snippet or URL: https://glitch.com/edit/#!/assets-dont-timeout-on-error?path=index.html%3A29%3A92

When a URL referred to by an asset is not resolvable, A-Frame scene loading just hangs forever.

See linked Glitch, which is a modified version of this https://aframe.io/examples/showcase/modelviewer/ but with the GLTF URL set to an invalid value.

One reason why it would be nice to initialize the scene, rather than hang forever, is that it would allow application-specific error handling.

I have a potential fix for this that seems to work, which I'll present as a PR.