TurboFan returned null handles if compilation did not succeed. This CL
changes that to a MaybeHandle to make it explicit that client code needs
to handle the error.

Bug: v8:7856
Change-Id: I6087e6263faa1150b9788213dd22c398b4a2fc2d
Reviewed-on: https://chromium-review.googlesource.com/1104688
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53824}