Fix 'selected frame' varobjs.

Fix 'selected frame' varobjs.
        * varobj.c (struct varobj): Remove the error field.
        (varobj_set_value): Don't check var->error.
        (install_new_value): Don't set var->error.
        (varobj_update): Always pass the new value
        of the root via install_new_value.
        (create_child): Don't set error field.
        (new_variable): Likewise.
        (c_value_of_root): Always reevaluate the value
        of selected frame varobjs in the selected frame.
        Don't call reinit_frame_cache.