Change incorrect subscribe return type to a GraphQLError rather than systems error

Replicates graphql/graphql-js@ea1894a

Author: Cito

src/graphql/execution/subscribe.py

@@ -145,18 +145,8 @@ async def create_source_event_stream(
         return ExecutionResult(data=None, errors=context)
 
     try:
-        event_stream = await execute_subscription(context)
-
-        # Assert field returned an event stream, otherwise yield an error.
-        if not isinstance(event_stream, AsyncIterable):
-            raise TypeError(
-                "Subscription field must return AsyncIterable."
-                f" Received: {inspect(event_stream)}."
-            )
-        return event_stream
-
+        return await execute_subscription(context)
     except GraphQLError as error:
-        # Report it as an ExecutionResult, containing only errors and no data.
         return ExecutionResult(data=None, errors=[error])
 
 
@@ -207,6 +197,13 @@ async def execute_subscription(context: ExecutionContext) -> AsyncIterable[Any]:
         if isinstance(event_stream, Exception):
             raise event_stream
 
+        # Assert field returned an event stream, otherwise yield an error.
+        if not isinstance(event_stream, AsyncIterable):
+            raise GraphQLError(
+                "Subscription field must return AsyncIterable."
+                f" Received: {inspect(event_stream)}."
+            )
+
         return event_stream
     except Exception as error:
         raise located_error(error, field_nodes, path.as_list())

tests/execution/test_subscribe.py

@@ -354,11 +354,16 @@ async def should_pass_through_unexpected_errors_thrown_in_subscribe():
     @mark.asyncio
     @mark.filterwarnings("ignore:.* was never awaited:RuntimeWarning")
     async def throws_an_error_if_subscribe_does_not_return_an_iterator():
-        with raises(TypeError) as exc_info:
-            await subscribe_with_bad_fn(lambda _obj, _info: "test")
-
-        assert str(exc_info.value) == (
-            "Subscription field must return AsyncIterable. Received: 'test'."
+        assert await subscribe_with_bad_fn(lambda _obj, _info: "test") == (
+            None,
+            [
+                {
+                    "message": "Subscription field must return AsyncIterable."
+                    " Received: 'test'.",
+                    "locations": [(1, 16)],
+                    "path": ["foo"],
+                }
+            ],
         )
 
     @mark.asyncio