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/test/preview-server/preview-error.spec.ts

@@ -0,0 +1,39 @@
+// Imports
+import {PreviewServerError} from '../../lib/preview-server/preview-error';
+
+// Tests
+describe('PreviewServerError', () => {
+  let err: PreviewServerError;
+
+  beforeEach(() => err = new PreviewServerError(999, 'message'));
+
+
+  it('should extend Error', () => {
+    expect(err).toEqual(jasmine.any(PreviewServerError));
+    expect(err).toEqual(jasmine.any(Error));
+
+    expect(Object.getPrototypeOf(err)).toBe(PreviewServerError.prototype);
+  });
+
+
+  it('should have a \'status\' property', () => {
+    expect(err.status).toBe(999);
+  });
+
+
+  it('should have a \'message\' property', () => {
+    expect(err.message).toBe('message');
+  });
+
+
+  it('should have a 500 \'status\' by default', () => {
+    expect(new PreviewServerError().status).toBe(500);
+  });
+
+
+  it('should have an empty \'message\' by default', () => {
+    expect(new PreviewServerError().message).toBe('');
+    expect(new PreviewServerError(999).message).toBe('');
+  });
+
+});