Fix PB List Keys to missing bucket type causes crash, hangs client

Per defect #875, list keys and buckets for an undefined bucket type failed to operate as expected.  Listing keys for an undefined bucket type caused a client hang while listing buckets for an undefined bucket type yielded an {ok, []}.  With this patch, both scenarios return {error, no_type}, and do not hang the client waiting for a reply.

This PR requires modifications to the riak_core_coverage_fsm provided by https://github.com/basho/riak_core/pull/588.