Emit SPICE_CHANNEL_ERROR_TLS when certificate can't be found
Emit SPICE_CHANNEL_ERROR_TLS when certificate can't be found

When trying to start remote-viewer with SPICE over TLS with
--spice-ca-file with a wrong filename, the connection fails
but remote-viewer keeps displaying the "Trying to connect"
message. The only hint that something went wrong is:
(remote-viewer:12924): GSpice-WARNING **: loading ca certs from a/home/teuf/foo.crt

This patch makes sure we emit a SPICE_CHANNEL_ERROR_TLS before
giving up on channel creation to inform the application that
an error happened.

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