We are currently storing compilation errors in the individual
compilation units and pass it to the ErrorThrower during finishing.
This CL changes that to store errors on the CompilationState directly.
From there, it is propagated to the ErrorThrower in the compilation
state callback.
This removes more work from the finisher task and slims down the
WasmCompilationUnits.

R=mstarzinger@chromium.org

Bug: v8:8343, v8:7921
Change-Id: Id332add43d4219d2a30fee653ed4e53a9b2698d9
Reviewed-on: https://chromium-review.googlesource.com/c/1303720Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57091}