Improve error message for bad return from subscribe resolver

After asserting AsyncIterable is required, show what was received instead.

Author: leebyron

src/subscription/__tests__/subscribe-test.js

@@ -585,7 +585,7 @@ describe('Subscribe', () => {
         invalidEmailSchema,
         ast
       );
-    }).to.throw('Subscription must return Async Iterable.');
+    }).to.throw('Subscription must return AsyncIterable. Received: test');
   });
 
   it('expects to have subscribe on type definition with iterator', () => {

src/subscription/subscribe.js

@@ -224,10 +224,12 @@ export function createSourceEventStream(
     throw subscription;
   }
 
-  invariant(
-    isAsyncIterable(subscription),
-    'Subscription must return Async Iterable.'
-  );
+  if (!isAsyncIterable(subscription)) {
+    throw new Error(
+      'Subscription must return Async Iterable. ' +
+        'Received: ' + String(subscription)
+    );
+  }
 
   return (subscription: any);
 }