Fix unrecoverable error when opening native db

[simolus3]

When opening a native database fails (for instance because the open factory is misconfigured and sends an invalid statement as a open pragma), we don't properly report this error to the client.

I would expect it to be reported through the initialize() future, but that actually misses the error. Also, the error is always reported as an unhandled exception which can easily crash the app.

This fixes these issues by:

1. Making SqliteDatabaseImpl.initialized report SqliteConnectionImpl.initialized instead of an always-completed future.
2. Avoid sending a close message if the isolate has crashed during open - we'll just get a guaranteed "already closed" error.
3. In SqliteoConnectionImpl, make isInitialized point to _open. Previously, _open was unawaited which caused an unhandled exception if an exception was thrown there.
4. In ParentPortClient, don't use sendPortFuture.then(). Attaching a first listener without an error handler used to also cause unhandled exceptions for reasons I'm not sure I understand. As an alternative we can set the SendPort field in the only place where initCompleter.complete is called.