Feature: add abort method to result object

[lindyhopchris]

This new method throws a FailedResultException if the result is a failure; otherwise it does nothing. This means the following:

$result = $bus->dispatch($command);

if ($result->didFail()) {
    throw new FailedResultException($result);
}

can be rewritten as:

$result = $bus->dispatch($command);
$result->abort();
// here result is definitely a success.

This PR also changes the ResultInterface::value() method to throw FailedResultException if attempting to get a value from a failed result. Previously it threw a ContractException - but the failed result exception makes more sense as it contains the originating result.