riak_core_worker_pool - reply on exit

[martinsumner]

When the riak_core_worker_pool exits it does not reply in a valid way (in 2.9):

{{function_clause,[{riak_core_vnode,reply,[<0.947.0>,{error,vnode_shutdown}],[{file,"src/riak_core_vnode.erl"},{line,1024}]},{riak_core_worker_pool,discard_queued_work,2,[{file,"src/riak_core_worker_pool.erl"},{line,263}]},{riak_core_worker_pool,terminate,3,[{file,"src/riak_core_worker_pool.erl"},{line,227}]},{gen_fsm,terminate,7,[{file,"gen_fsm.erl"},{line,597}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]},[{gen_server,terminate,6,[{file,"gen_server.erl"},{line,744}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}

The Work is From just a PID, and so this is not a valid pattern match on riak_core_vnode:reply/2