When an allocation is garbage collected from the client, but not from
the servers, the API request is routed to the client and the client
does attempt to read the file, but the alloc dir has already been
deleted, resulting in a 500 error.

This happens because the client GC only destroys the alloc runner
(deleting the alloc dir), but it keeps a reference to the alloc runner
until the alloc is garbage collected from the servers as well.

This commit adjusts this logic by checking if the alloc runner (and the
alloc files) has been destroyed, returning a 404 if so.