FEATURE raise errors, because the client won't

Most other client raise errors when error are encountered. For example, Memcache will do so when the value is too large.

Both tokyotyrant gems do not raise, so I've added code to check if an error has occurred.

#store and #delete will return false if the action failed.
#load will return nil if the record doesn't exist, or if an error occurred.
... or error