The rejection value must be an Exception

[clue]

The following code is not supported:

$promise = React\Promise\reject(123);

$blocker = new Blocker($loop);

// will fail while trying to `throw 123`
$blocker->await($promise);

At the very least, we should add some documentation.

In the future, we should consider wrapping primitive values in some special Exception sub-class.

[clue]

See also reactphp/promise#46