streams: Display error if stream update fails.

Displays a toast containing an error string if a call to
updateExistingStream returns a failed promise.

Fixes: #5286

src/streams/EditStreamScreen.js

@@ -8,6 +8,7 @@ import * as NavigationService from '../nav/NavigationService';
 import { useSelector, useDispatch } from '../react-redux';
 import { updateExistingStream, navigateBack } from '../actions';
 import { getStreamForId } from '../selectors';
+import { showToast } from '../utils/info';
 import Screen from '../common/Screen';
 import EditStreamCard from './EditStreamCard';
 
@@ -22,7 +23,11 @@ export default function EditStreamScreen(props: Props): Node {
 
   const handleComplete = useCallback(
     (name: string, description: string, isPrivate: boolean) => {
-      dispatch(updateExistingStream(stream.stream_id, stream, { name, description, isPrivate }));
+      dispatch(
+        updateExistingStream(stream.stream_id, stream, { name, description, isPrivate }),
+      ).catch(error => {
+        showToast(error.message);
+      });
       NavigationService.dispatch(navigateBack());
     },
     [stream, dispatch],