Fix dialyzer error in kv_mrc_map

This was uncovered once basho/riak_pipe
a33ac811ddefff0f0a9c04cbec4611a1537ed98a was committed.
Merge pull request #941 from basho/bugfix/pipe-worker-init-spec

Fix dialyzer error in kv_mrc_map

Reviewed-by: seancribbs
riak_kv_env:doc_env/0 handles the case of an erlang that doesn't support
 erlang:system_info(ets_limit)
Merge pull request #942 from basho/bugfix/jnd/everybody-to-the-limit

Better error handling from basho/riak_kv#935

Reviewed-by: reiddraper