No place to reject layer creation in algorithms

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.

Read sections 6.1 and 12.2 of the WebXR Layers specification, then trace the layer-creation algorithms referenced there. Determine where rejection for unsupported layers or resource limits should be specified and what completion criteria the surrounding specification requires; the issue does not identify a source file or test.