Correctly mark unhandled exceptions as `handled: false`

[cleptric]

We exposed the ExceptionMechanism via the EventHint, so we can now correctly set the handled property in the SDK's error handler.

Replace all calls to captureException() with

$hint = EventHint::fromArray([
    'exception' => $exception,
    'mechanism' => new ExceptionMechanism(ExceptionMechanism::TYPE_GENERIC, false),
]);

captureEvent(Event::createEvent(), $hint);

getsentry/sentry-php#1416