And remove the TurboFan/Liftoff specific {FinishCompilation}
implementations completely. Compilation errors are now stored in the
{WasmCompilationUnit} directly as a {Result<WasmCode*>}. They are
retrieved via {WasmCompilationUnit::ReportError}, which moves the error
to the {ErrorThrower}.
This prepares more changes to completely remove the {FinishCompilation}
phase.

R=titzer@chromium.org

Bug: v8:7921
Change-Id: I4f9a6e919359aeab074880d0d38211500b76e4ec
Reviewed-on: https://chromium-review.googlesource.com/c/1290975
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56826}