file-xfer: inform agent of errors only when task finished
file-xfer: inform agent of errors only when task finished

No need to inform of a problem under
spice_file_transfer_task_completed() as the task will be finalized and
we can send the error to the agent there.

This change is related to split SpiceFileTransferTask from
channel-main.

Acked-by: Jonathon Jongsma <c7254805a17bd5c41e24c3a278fcbf3afcdfb0fa@redhat.com>

fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk