(kkc-lookup-cache): Initialize it to nil.

(kkc-lookup-cache): Initialize it to nil.
(kkc-lookup-cache-tag): New constant.
(kkc-lookup-key): If kkc-lookup-cache is nil, initialize it.  Use
kkc-init-file-name.
(kkc-region): Fix previous change.  Call kkc-error on error.
(kkc-shorter-conversion, kkc-longer-phrase): New functions.
(kkc-keymap): Bind them to "I" and "O" respectively.
(kkc-error): New error symbol and new function.
(kkc-longer, kkc-shorter): Call kkc-error on error.
(kkc-show-conversion-list-or-next-group): Likewise.
(kkc-show-conversion-list-or-prev-group): Likewise.