Invalid transaction return value when a vhost is removed while a policy is updated

[dumbbell]

In the update_polices() function in rabbit_policy.erl:

    {Xs, Qs} = rabbit_misc:execute_mnesia_transaction(
                 fun() ->
                         [mnesia:lock({table, T}, write) || T <- Tabs], %% [1]
                         case catch list(VHost) of
                             {error, {no_such_vhost, _}} ->
                                 ok; %% [2]
                               % ^^

The transaction function returns ok if it couldn't query the list of policies for the given vhost. However, the calling code expects a tuple.

I discovered it while working on #744, though I didn't try to reproduce it yet.

[dumbbell]

This will be fixed in pull request #745.