Avoid sending user errors to client (vercel#8005)

* Avoid sending user error to client Closes vercel#7526

* Small improvments

* Update sending errors

packages/next-server/server/api-utils.ts

@@ -49,7 +49,8 @@ export async function apiResolver(
     if (e instanceof ApiError) {
       sendError(res, e.statusCode, e.message)
     } else {
-      sendError(res, 500, e.message)
+      console.error(e)
+      sendError(res, 500, 'Internal Server Error')
     }
   }
 }
@@ -226,7 +227,7 @@ export function sendError(
 ) {
   res.statusCode = statusCode
   res.statusMessage = message
-  res.end()
+  res.end(message)
 }
 
 interface LazyProps {

packages/next-server/server/next-server.ts

@@ -313,7 +313,7 @@ export default class Server {
       }
     }
 
-    apiResolver(
+    await apiResolver(
       req,
       res,
       params,

test/integration/api-support/pages/api/user-error.js

@@ -0,0 +1,3 @@
+export default (req, res) => {
+  throw new Error('User error')
+}

test/integration/api-support/test/index.test.js

@@ -53,6 +53,13 @@ function runTests (serverless = false) {
     expect(json).toEqual({ error: 'Server error!' })
   })
 
+  it('should throw Internal Server Error', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/user-error', null, {})
+    const text = await res.text()
+    expect(res.status).toBe(500)
+    expect(text).toBe('Internal Server Error')
+  })
+
   it('should parse JSON body', async () => {
     const data = await fetchViaHTTP(appPort, '/api/parse', null, {
       method: 'POST',