toThrow should return the error

## 🚀 Feature Proposal

`.toThrow` and `.rejects.toThrow` should return the error thrown.

## Motivation

When working with a project that uses ava I noticed their [`.throws`](https://github.com/avajs/ava/blob/master/docs/03-assertions.md#throwsfn-expected-message) and [`.throwsAsync`](https://github.com/avajs/ava/blob/master/docs/03-assertions.md#throwsasyncthrower-expected-message) return the original error. It is very convenient.

This would make it possible to never need the [`expect.hasAssertions()` + `try / catch`](https://github.com/jest-community/eslint-plugin-jest/issues/295#issuecomment-509974545) syntax.

## Example

```js
function throwSyncError() {
const syncError = new Error('sync error');
syncError.code = 'SYNC';
throw syncError;
}

async function throwAsyncError() {
const asyncError = new Error('sync error');
asyncError.code = 'ASYNC';
throw asyncError;
}

test('can get error message', async () => {
const syncError = expect(() => throwSyncError()).toThrow('sync error');
expect(syncError.code).toEqual('SYNC');

const asyncError = await expect(throwAsyncError()).rejects.toThrow('sync error');
expect(asyncError.code).toEqual('ASYNC');
});
```

## Pitch

Because using `.toThrow` and `.rejects.toThrow` over `try/catch` it prevents tests that don't fail because they no longer reject. https://github.com/facebook/jest/issues/3917

Start by locating Jest's synchronous `.toThrow` and promise `.rejects.toThrow` matcher entry points and their tests. Use the proposal's sync and async examples to understand the expected returned values, including the custom `code` property. Done means both matchers return the original thrown errors while preserving their existing assertion behavior.

Written by the indexing model from the issue text.