Remove boilerplate code from api error handling

If you look at the code for views in horizon dashboard, you will see a lot of this pattern repeating everywhere:

        flavors = []
        try:
            flavors = api.nova.flavor_list(request, None)
        except Exception:
            exceptions.handle(request, _('Unable to retrieve flavor list.'))

This code would be repeated everywhere, where api.nova.flavor_list is called, with the same exact error message and the same default value.

It would be nice to refactor that, for example using a decorator, like we did in Tuskar-UI: https://github.com/openstack/tuskar-ui/blob/master/tuskar_ui/handle_errors.py

Then the default error message and value can be specified once, on the api method itself, and later overriden if needed.

The above code then turns into:

    @handle_errors(_('Unable to retrieve flavor list.'), [])
    def flavor_list(...):...
    ...
    api.nova.flavor_list(request, None)

We can also use the decorator in-place explicitly, without having decorated the method:

    handle_errors(_('Unable to retrieve flavor list.'), [])(api.nova.flavor_list)(request, None)