main: to let SpiceFileTransferTask handle errors
main: to let SpiceFileTransferTask handle errors

* on VD_AGENT_FILE_XFER_STATUS_CAN_SEND_DATA, if the file-transfer is
  on pending state, spice_file_transfer_task_read_async() will call the
  callback with error set.

* on VD_AGENT_FILE_XFER_STATUS_SUCCESS, if the file-transfer is on
  pending state, spice_file_transfer_task_completed() will set the
  error for this task.

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

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