No place to reject layer creation in algorithms

[alcooper91]

There's a couple of bits of text that indicate that sessions may reject layers for various reasons[1][2], but there's no indication of a spot and/or type of exception that should be thrown in the creation algorithm.

[1] At the end of section 6.1:

Some layer types may not be supported by the XRSession. If a layer type isn’t supported the method will throw an exception. XRProjectionLayer MUST be supported by all XRSessions.

[2] Section 12.2:

The user agent MAY put limits on any resource allocation such as the maximum pixel size or the number of layers to reduce the identifiability of the GPU hardware.

Note that for [2], no prose is written for either restricting the size or due to the number of layers.

[cabanier]

There's a couple of bits of text that indicate that sessions may reject layers for various reasons[1][2], but there's no indication of a spot and/or type of exception that should be thrown in the creation algorithm.

[1] At the end of section 6.1:

Some layer types may not be supported by the XRSession. If a layer type isn’t supported the method will throw an exception. XRProjectionLayer MUST be supported by all XRSessions.

I see that the algorithm to create the layers contains:

If session was not created with "[layers](https://immersive-web.github.io/layers/#feature-descriptor-layers)" enabled, throw a [NotSupportedError](https://webidl.spec.whatwg.org/#notsupportederror) and abort these steps.

Do I need to add it to other locations?

[2] Section 12.2:

The user agent MAY put limits on any resource allocation such as the maximum pixel size or the number of layers to reduce the identifiability of the GPU hardware.

Note that for [2], no prose is written for either restricting the size or due to the number of layers.

I'll work on getting those added

[alcooper91]

There's a couple of bits of text that indicate that sessions may reject layers for various reasons[1][2], but there's no indication of a spot and/or type of exception that should be thrown in the creation algorithm.
[1] At the end of section 6.1:

Some layer types may not be supported by the XRSession. If a layer type isn’t supported the method will throw an exception. XRProjectionLayer MUST be supported by all XRSessions.

I see that the algorithm to create the layers contains:

If session was not created with "[layers](https://immersive-web.github.io/layers/#feature-descriptor-layers)" enabled, throw a [NotSupportedError](https://webidl.spec.whatwg.org/#notsupportederror) and abort these steps.

Do I need to add it to other locations?

As written it makes it seem like even with the layers feature supported a UA could opt not to support all layer types (e.g. maybe it doesn't support a Quad layer but does support an Equirect layer). If this is unintentional (which I guess it may be, since I think we've kind of viewed it as "Either you support only projection layers or you support ALL layers"), maybe we should just tighten up the language about what a "layer not supported" means to match more of the spec text?