chore(migration): add better error message when request fails (#5567)

* chore(migration): add better error message when request fails

* fix(migration): add error message if json parsing fails

packages/@sanity/migrate/src/fetch-utils/fetchStream.ts

@@ -8,17 +8,25 @@ export interface HTTPError extends Error {
   statusCode: number
 }
 
-export function assert2xx(res: Response) {
+export async function assert2xx(res: Response): Promise<void> {
   if (res.status < 200 || res.status > 299) {
-    const err = new Error(`HTTP Error ${res.status}: ${res.statusText}`) as HTTPError
+    const response = await res.json().catch(() => {
+      throw new Error(`Error parsing JSON ${res.status}: ${res.statusText}`)
+    })
+
+    const message = response.error
+      ? response.error.description
+      : `HTTP Error ${res.status}: ${res.statusText}`
+
+    const err = new Error(message) as HTTPError
     err.statusCode = res.status
     throw err
   }
 }
 
 export async function fetchAsyncIterator({url, init}: FetchOptions) {
   const response = await fetch(url, init)
-  assert2xx(response)
+  await assert2xx(response)
   if (response.body === null) throw new Error('No response received')
   return streamAsyncIterator(response.body)
 }