This PR addresses an issue with the error serialization mentioned in #6571.

After some digging, it turns out that there are several XML-services (rest-xml, query) which define default members in their error shapes (f.e. cloudfront, route53, cloudwatch, iam,...). These default members are not added to the root node, while non-default members are.

For JSON-services (json, rest-json), there is no sub-element for the error metadata. All members are added to the root node. Here, defined default members in the spec can modify the casing (f.e. Message in DynamoDB's TransactionCanceledException).

The following changes are contained in this PR:

• Enhance the error serialization for error shapes which define additional members which "conflict" with the default error fields (like "message" or "code").
• Remove the member "type" from the special handling (this was a wrong assumption in the past, it doesn't correlate with the code, only __type for json protocols do). F.e. in Lambda, the "Type" field on exceptions is "SOAP"-like and contains the value "User" to indicate that it's a user-triggered error.
• Fix the scaffold to also generate an additional "Type" field (f.e. for Lambda).
• Regenerate the APIs (since the scaffold was modified).
• Clean up some tests and added some more.