[cli][node] make error handling of edge functions consistent with serverless functions in vc dev (#8007)

When edge functions error, they were showing a basic text response instead of the error template. They were also sending a 502 status code instead of a 500.

This PR makes the error handling of Edge Functions consistent with Serverless Functions when executed through `vc dev`. If we want to update the error templates themselves, we can do that in a separate PR.

**Note:** Production currently treats Edge Function errors differently from Serverless Function errors, but that's a known issue that will be resolved.

---

*I deleted the original outputs (terminal and browser screenshots) because they are out of date and added a lot of content to this page. See my latest comment for updated examples.*

packages/cli/test/dev/utils.js

@@ -257,6 +257,10 @@ async function testFixture(directory, opts = {}, args = []) {
   dev.kill = async (...args) => {
     dev._kill(...args);
     await exitResolver;
+    return {
+      stdout,
+      stderr,
+    };
   };
 
   return {