Structured Error Output

[AndrewAsseily]

Issue #, if available: CLI-5136 & CLI-7572 & https://github.com/aws/aws-cli/issues/2688

Description of changes:

Exposes modeled error members from AWS service exceptions to stdout in the configured output format (json/yaml/text/table/off), while maintaining the existing formatted error message on stderr.

Behavior:

Command: aws s3api get-object --bucket not-a-real-bucket-0000 --key file.txt out.txt

Before:

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

After (stdout):

{
    "Code": "NoSuchBucket",
    "Message": "The specified bucket does not exist",
    "BucketName": "not-a-real-bucket-0000"
}

After (stderr):

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

Configuration:

Users can opt out via:

• Config: cli_error_format = LEGACY
• Environment: AWS_CLI_ERROR_FORMAT=LEGACY

Additional Changes:

• Added off as a valid output format option to suppress all output

Description of tests:

• Unit and functional tests pass locally.