wasm::ErrorThrower doesn't actually throw exceptions, it just schedules them.
As a result, this exception isn't handled properly by code which expects
ValueDeserializer to actually throw. For instance, the unit tests use a
TryCatch to catch and handle expected exceptions in unit tests.

Before this patch, I see local unit test failures because a wasm decode test
schedules one, but it isn't caught (and instead causes Context::New to fail
at the beginning of the next test).

BUG=685713

Review-Url: https://codereview.chromium.org/2659483004
Cr-Commit-Position: refs/heads/master@{#42718}