Add ErrorCodeOverride to error structs

[eddy-aws]

Description of changes:
Makes changes to AwsJson protocol generator.

• Introduces ErrorCodeOverride field to error structs
• Returns ErrorCodeOverride if not empty. Else returns default error code
• Introduces generateErrorShape method to allow override default error block for unmodeled errors

For example, AWS code bases would have something like the following to override generateErrorShape:

    @Override
    protected Set<StructureShape> generateErrorShapes(
        GenerationContext context, OperationShape operation, Symbol responseType) {
        if (isAwsQueryCompatibleTraitFound(context)) {
            return HttpProtocolGeneratorUtils.generateErrorDispatcher(
                context, operation, responseType, this::writeErrorMessageCodeDeserializer,
                this::getOperationErrors, (writer) -> AwsJsonRpc1_0.awsQueryCompatibleDefaultBlockWriter(writer));
        } else {
            return HttpProtocolGeneratorUtils.generateErrorDispatcher(
                context, operation, responseType, this::writeErrorMessageCodeDeserializer,
                this::getOperationErrors);
        }
    }

    private static void awsQueryCompatibleDefaultBlockWriter(GoWriter writer) {
        writer.openBlock("default:", "", () -> {
            writer.openBlock("genericError := &smithy.GenericAPIError{", "}", () -> {
                    writer.write("Code: getAwsQueryErrorCode(response),");
                writer.write("Message: errorMessage,");
            });
            writer.write("return genericError");
        });
    }

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[syall]

There should not be any AWS-specific code in generic smithy code generators.

[isaiahvita]

agree with @syall, AWS logic should be in the codegen module in the V2 package. only generic service (not AWS specific) code generation logic should be in smithy-go

[isaiahvita]

just to clarify the intent behind:

Introduces generateErrorShape method to allow override default error block for unmodeled errors

It looks like the plan is in service-specific codebases (in our case, the aws codebase) to override generateErrorShapes to call the newgenerateErrorDispatcher with a different DefaultBlockWriter, and the reason you made the existing generateErrorDispatcher function public is so that the overloaded functions can having matching privacy annotations?

[eddy-aws]

just to clarify the intent behind:

Introduces generateErrorShape method to allow override default error block for unmodeled errors

I have updated the description with some additional details

[syall]

Rebased changes, but will need to Squash commits before merging.