gnutls: Provide better error message when TLS alert is received
gnutls: Provide better error message when TLS alert is received

Print a message describing the alert.

This also changes the error code used when a TLS alert is received from
G_TLS_ERROR_NOT_TLS, which is clearly inappropriate as the peer must
support TLS if it is sending a TLS alert, to G_TLS_ERROR_MISC.

https://bugzilla.gnome.org/show_bug.cgi?id=782218

GNOME/glib-networking,GNOME/glib-networking,GNOME/glib-networking