This currently runs into UNIMPLEMENTED. Since the fuzzer starts hitting
this, make the error message a bit nicer to spot this more easily.

R=ahaas@chromium.org

Bug: chromium:1035233
Change-Id: I63caac37e9b767fba9d6e4b71cb54a8a77b87ab9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1991493Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65655}