It was only set by ha_innobase::prepare_drop_index(), which can return
the appropriate MySQL error code (HA_ERR_DROP_INDEX_FK) directly.

ha_innobase::add_index(): Correct the function comment.

ha_innobase::prepare_drop_index(): Correct the function comment.
Return MySQL error codes directly.

ha_innobase::final_drop_index(): Correct the function comment.