add response body string to unknown error message

If incoming error response is unknow error, we only make best effort
to infer the error type. This change puts error response body to error
message so that users have the information they need. This change is
necessary for Node because when error response is unknown, the body
stream will never be consumed, which would use a lot of resources
before stream is flushed.

Author: AllanZhengYP

smithy-typescript-codegen/src/main/java/software/amazon/smithy/typescript/codegen/integration/HttpProtocolGeneratorUtils.java

@@ -203,10 +203,11 @@ static Set<StructureShape> generateErrorDispatcher(
                 // Build a generic error the best we can for ones we don't know about.
                 writer.write("default:").indent()
                         .write("errorCode = errorCode || \"UnknownError\";")
-                        .openBlock("response = {", "};", () -> {
+                        .openBlock("response = {", "} as any;", () -> {
                             writer.write("__type: `$L#$${errorCode}`,", operation.getId().getNamespace());
                             writer.write("$$fault: \"client\",");
                             writer.write("$$metadata: deserializeMetadata(output),");
+                            writer.write("message: await collectBodyString(output.body, context)");
                         }).dedent();
             });
             writer.write("return Promise.reject(Object.assign(new Error(response.__type), response));");