React version: 0.0.0-experimental-94c0244ba

## Steps To Reproduce

Mutating the external source during the initial render before subscribe throws error. 

https://codesandbox.io/s/usemutablesource-7nbs6?file=/src/App.js

![image](https://user-images.githubusercontent.com/8140606/94963407-f7033280-04ac-11eb-9cf7-e00c6630f55e.png)

1. Navigate to the above code sandbox
2. Click on the Mutate button. Final Value is rendered without any error. (Success Mode)
3. Toggle to Error Mode and click on the Mutate button again.
4. Error thrown (and the UI is rendered with the Final Value when run from vscode)

> Note: In our actual codebase (private), we are seeing this error at different instances e.g. component getting updated due to `setState` call in `useLayoutEffect` etc.

## The current behavior

Seeing errors in development build.

![image](https://user-images.githubusercontent.com/8140606/94962796-e3a39780-04ab-11eb-82d9-b24e4eaa5cc1.png)

## The expected behavior

These errors thrown by useMutableSource should not showup on the UI and not passed to the error boundary as the component is eventually re-rendered with the latest state without any error.
/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import type {
  MutableSource,
  MutableSourceGetSnapshotFn,
  MutableSourceSubscribeFn,
  ReactContext,
} from 'shared/ReactTypes';