Error handling in riak_core_coverage_fsm:init

Adds error handling the riak_core_coverage_fsm:init to stop the FSM when the Mod:init function call returns {stop, _}.  This change supports the for https://github.com/basho/riak_kv/issues/875 (PB List Keys to missing bucket type causes crash, hangs client).

This PR replaces https://github.com/basho/riak_core/pull/588 to squash commits.  Due to a mistake rebasing to squash, I simply created a new branch with the few relevant changes.