Address code review comments/concerns

Update Makefile to check for rebar, as well as use less strict dialyzer
parameters by default.

Make test/sc.erl compile clean without warnings so that 'make test' works.

Fix an 'ummatched return' dialyzer complaint.

Add warning for corrupted riak_ensemble_basic_backend state.

Remove catch-all case for riak_ensemble_router:fail_cast.