missing informative way of extracting WebGL context creation failures

as discussed [here](https://github.com/emscripten-core/emscripten/pull/11441#pullrequestreview-435967112), today the only information that emscripten returns about WebGL context creation failures appears in logs, if the `GL_DEBUG` or `ASSERTIONS` flags are set during the build process. There's no programatic way of receiving the source of failure in `GL.createContext` or `emscripten_webgl_create_context`, which means that these failures can't be handled in real time, and any logs must be extracted from the console - both of which are problematic when shipping code for production.
Something like the proposed `emscripten_webgl_create_context_ex` should be created, preferably with the ability to extract error information from also from the internal call to `GL.createContext`.