The entry points to the deserializer are responsible for ensuring that an
exception is pending by the time they return. Some failures throw exceptions
themselves, while others (like errors in the format) are exceptions caused by
the deserializer, not coming from the runtime.

Like the non-legacy path, a default deserialization exception should be thrown
in such cases.

BUG=chromium:693411

Review-Url: https://codereview.chromium.org/2712713002
Cr-Commit-Position: refs/heads/master@{#43390}