[wasm] Cleanup and modularize mjsunit/wasm/exceptions.

R=ahaas@chromium.org
TEST=mjsunit/wasm/exceptions

Change-Id: I6301cf9bcb94e972f4e7eb3d096c650a5afc55ca
Reviewed-on: https://chromium-review.googlesource.com/1190343
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55453}