There are wasm tests which are supposed to fail, but they fail for the
wrong reason (because blocks do not declare their type).
This CL fixes this, and changes the error output to contain the
expected and observed results for better debuggability.

R=titzer@chromium.org

Change-Id: Ibbd7883e43677a91f858575578586c9e9d1641d5
Reviewed-on: https://chromium-review.googlesource.com/571810Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46698}