Throw error if mock not found

Author: dlarocque

packages/vertexai/test-utils/mock-response.ts

@@ -58,6 +58,9 @@ export function getMockResponseStreaming(
 
 export function getMockResponse(filename: string): Partial<Response> {
   const fullText = mocksLookup[filename];
+  if (!(filename in mocksLookup)) {
+    throw Error(`Mock response file '${filename}' not found.`);
+  }
   return {
     ok: true,
     json: () => Promise.resolve(JSON.parse(fullText))