Propagate errors during rendering WebRender instead of panicking

[sotaroikeda]

#768 propagates errors when initializing WebRender. But it is not enough to propagate errors instead of panicking, since actual shader compilations happen when they are used under Renderer::render() in normal use cases. Then the render() panics when the compilations failed. gecko want to avoid the panic.

The problem of gecko is tracked by Bug 1390138