Improve file transfer error messages
Improve file transfer error messages

In preparation for potentially displaying error messages to a user in a
UI, I thought I'd improve the messages slightly.

Acked-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

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