Add protection against folsom stat errors

Folsom may sometimes return an error tuple if something goes wrong (see
folsom_ets.erl), but our code was only catching exceptions. So the error
would end up being used as a valid value and crash the riak_kv_stat
process later. This fixes that problem and gives us better protection
from folsom funkiness.
Merge pull request #356 from basho/eas-folsom-stat-error-protection

Add protection against folsom stat errors