Check value of QFile::{open,flush,write}() when saving messages/attachments
To achieve this, a couple of things happened:
* At every place where an instance of FileDownloadManager is created,
the transferError() signal from FileDownloadManager is connected to
the transferError() signal from MessageView.
* The transferError() signal from MessageView is connected to the
slotDownloadTransferError() slot from MainWindow (was
slotDownloadMessageTransferError() first).
Thanks to Thomas and Jan for their feedback.