Refactor smithy_types::Error to be more flexible (Breaking Change) (#426)

* Refactor smithy_types::Error to be more flexible

S3 (and probably other) services have errors with an additional set of metadata that must be recorded. This diff does a few things:
1. Make the internal fields of smithy_error::Error private (as they should have been from the beginning)
2. Adds a builder to easily construct errors
3. Adds a property bag to enable storing additional fields on the error.

This will make it straightforward to add per-service customizations to record additional error metadata.

* Fix two missed refactorings