Make glib::value::GetError fields publics

[fengalin]

... so that we can read them outside of the module and we can build an error for use in assert_eq! expressions.

[sdroege]

Or a constructor? Either way seems fine to me

[fengalin]

Yeah, I'm not sure yet. We don't want users to change the fields values, but OTOH they might want to read the fields.

[fengalin]

Of course adding a constructor is useful, there's no question about this. One argument in favour of setting the fields as public is that when we declare an Error based on an enum, the variants fields are public.