file-xfer: non-programming errors should be reported in async
file-xfer: non-programming errors should be reported in async

It is fine to not return async errors for programming errors via
g_return_if_fail() and friends, however, we need to return proper
error if it's a normal run-time error.

elmarco/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,guodong/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk