ci(docs-infra): rename 'upload-server' to 'preview-server'

The server no longer has files uploaded to it. Instead it is more
accurate to refer to it as dealing with "previews" of PRs.

aio/aio-builds-setup/dockerbuild/scripts-js/lib/preview-server/utils.ts

@@ -0,0 +1,29 @@
+import * as express from 'express';
+import {promisify} from 'util';
+import {PreviewServerError} from './preview-error';
+
+/**
+ * Update the response to report that an error has occurred.
+ * @param res The response to configure as an error.
+ * @param err The error that needs to be reported.
+ */
+export async function respondWithError(res: express.Response, err: any): Promise<void> {
+  if (!(err instanceof PreviewServerError)) {
+    err = new PreviewServerError(500, String((err && err.message) || err));
+  }
+
+  res.status(err.status);
+  await promisify(res.end.bind(res))(err.message);
+}
+
+/**
+ * Throw an exception that describes the given error information.
+ * @param status The HTTP status code include in the error.
+ * @param error The error message to include in the error.
+ * @param req The request that triggered this error.
+ */
+export function throwRequestError(status: number, error: string, req: express.Request): never {
+  const message = `${error} in request: ${req.method} ${req.originalUrl}` +
+                  (!req.body ? '' : ` ${JSON.stringify(req.body)}`);
+  throw new PreviewServerError(status, message);
+}