an operator for ignoring any exceptions

[Hikaru Nakashima]

I think this idea is useful in async function.

For exsample, we write codes as below, when we use fetch()  in async
function.

```js

let res, text

try {
    res = await fetch( url )
} catch ( err ) { console.log( 'network error' ) }

if ( ! res.ok ) console.log( 'server error' )
else text = await res.text( )


```

or

```js

let res, text

res = await fetch( url ).catch( err => null )
if ( ! res ) console.log( 'network error' )

else if ( ! res.ok ) console.log( 'server error' )
else text = await res.text( )


```

but, we can write as below  if we use this proposal.

```js

let res, text

res = try await fetch( url )
if ( ! res ) console.log( 'network error' )

else if ( ! res.ok ) console.log( 'server error' )
else text = await res.text( )

```

or do we need another syntax like "await?" ?
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