Changes:
- Simplify and generalize ToValueTypeString.
- Fix some error messages in msjunit so that they reflect the underlying
  error better.
- Change 'exn' -> 'exnref' to match exception-handling proposal.

Bug: v8:7581
Change-Id: I264f6c9aa598a57f39d5a4d01399af64db83a2b9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2243214
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68364}