Throw ObjectDisposedException and ArgumentNullException synchronously

[flagbug]

Severe exceptions like ObjectDisposedExceptions and AgumentNullException shouldn't OnError the observable, and should rather be thrown immediately/synchronously.

See https://github.com/akavache/Akavache/blob/master/Akavache.Sqlite3/SqlitePersistentBlobCacheNext.cs#L63 for an example where they are passed via OnError