GLTFLoader: Make error when loading textures with no data clear

[zeux]

If GLTFLoader is used to load a texture with no uri or bufferView, it
uses an empty uri which results in FileLoader loading the HTML document
from which the call originates. The resulting document is then fed into
an actual image loader such as KTX2Loader or BitmapLoader, which tends
to generate hard to understand errors.

This change detects the error early and throws a special message to make
debugging easier.

[donmccurdy]

What is a scenario where this error might occur? I don't think we'd want to go out of the way to include English error messages in the bundle to warn about malformed assets — there are an unending number of ways the asset might be invalid — but if this could indicate user error, like failing to register an extension handler, then I'm in favor of the change.

[zeux]

The scenario here is that either the source glTF file is invalid, or the image data comes from a non-optional image extension that three.js doesn't implement. (note that files with unknown required extensions aren't rejected)

I agree that we don't necessarily need to guard against every possible issue like that but I found the actual flow here (loading empty URI results in reinterpreting HTML data as image contents) very surprising, hence the change.

[mrdoob]

Thanks!