Metal: Fix a race in failing compilation subtasks Program link invokes two link subtasks, one MSL compilation per shader in program. If these tasks would fail, they would race to write the error message to the error string held in link object. Fix by recording the error message to the compile tasks and forwarding it to the context when sub task result is fetched. Bug: angleproject:392652319 Change-Id: Icb369f7bd62da5155d7915f2743ac53d91d61ea8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6207769 Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>