In the wasm streaming decoder error position test, do also check the
error messages generated. This revealed messages that were not quite
fitting and some that were formatted differently than the majority.

R=ahaas@chromium.org

Bug: v8:8814
Change-Id: If157f1083a104413bf14797ac56e756baac98c17
Reviewed-on: https://chromium-review.googlesource.com/c/1463780
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59604}