actually `use` exception classes

this is to prevent errors like this:

Can't locate object method "throw" via package "ShardedKV::Error::ReadFail" (perhaps you forgot to load "ShardedKV::Error::ReadFail"?) at [snip]/ShardedKV/Storage/MySQL.pm line 344.
Merge pull request #1 from ksurent/master

actually `use` exception classes