pushErrorScope method

Starts catching errors of one kind instead of letting them reach the console.

Without this pair nothing a browser rejects can ever be thrown in Dart. WebGPU validates asynchronously: createRenderPipeline returns a pipeline object whether or not the descriptor was legal, and the complaint arrives later, on the console, in a message the Dart program never sees. So a backend that means to report a refusal — which is what the conformance suite's refusal checks watch for — has to bracket the call. gpuChecked is that bracket, and is what a caller should reach for rather than this pair directly.