Detect constraints on event stream errors (#2069)

Constrained shapes in the closure of an event stream are not supported
[0]. However, the `ValidateUnsupportedConstraints` validator was not
detecting constrained shapes in event stream errors because the
`EventStreamNormalizer` model transformer pulls them out of the
`@streaming` union shape.

This commit makes it so that the validator will detect such usages, by
leveraging the `SyntheticEventStreamUnionTrait` trait that gets attached
in the model transformer.

[0]: https://github.com/awslabs/smithy/issues/1388



Co-authored-by: Luca Palmieri <20745048+LukeMathWalker@users.noreply.github.com>