Filter unmodeled fields from error output

[AndrewAsseily]

Description of changes: The CLI's "Additional error details" section previously displayed all fields from the error response, including fields not defined in the service model. This change filters the output to only show fields that are explicitly defined in the service's error shape model.

Non-modeled: https://github.com/aws/aws-cli/blob/v2/awscli/botocore/data/s3/2006-03-01/service-2.json#L8516

Before:

(.venv)  aws-cli % aws s3api get-object --bucket not-a-real-bucket-0000 --key file.txt out.txt

aws: [ERROR]: An error occurred (NoSuchBucket) when calling the GetObject operation: The specified bucket does not exist

Additional error details:
BucketName: not-a-real-bucket-0000

After:

(.venv)  aws-cli % aws s3api get-object --bucket not-a-real-bucket-0000 --key file.txt out.txt                          

aws: [ERROR]: An error occurred (NoSuchBucket) when calling the GetObject operation: The specified bucket does not exist

Modeled: https://github.com/aws/aws-cli/blob/v2/awscli/botocore/data/lambda/2015-03-31/service-2.json#L7930

(.venv) aws-cli % aws lambda get-function --function-name nonexistent-function-12345

aws: [ERROR]: An error occurred (ResourceNotFoundException) when calling the GetFunction operation: Function not found: arn:aws:lambda:us-west-2:<redacted-for-this-PR>:function:nonexistent-function-12345

Additional error details:
Type: User

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[ashovlin]

Can we do this through either the constructor or a property that defaults to none so we don't need to be as defensive with the getattr(exception, 'modeled_fields', None) later?

[ashovlin]

We're extracting .modeled_fields from the exception, copying that into error_info['_modeled_fields], then later using that to delete keys from error_info?

That was tough to follow. Can _extract_error_response, when it's building error_dict, ignore the unmodeled keys initially?

I think that'll be simpler and easier to follow. _extract_error_response remains responsible for extracting the info from the exception we care about, rather than that "leaking" that responsibility into a "display" method.