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/preview-error.ts

@@ -0,0 +1,8 @@
+// Classes
+export class PreviewServerError extends Error {
+  // Constructor
+  constructor(public status: number = 500, message?: string) {
+    super(message);
+    Object.setPrototypeOf(this, PreviewServerError.prototype);
+  }
+}