lib-index: mail_transaction_log_view_set() - Return -1 only on I/O error

lib-index: mail_transaction_log_view_set() - Return -1 only on I/O error

The callers that care about the difference:

 * mail_index_sync_map() already assumes that -1 means I/O error
 * mail_index_sync_set_log_view() handles 0 by logging an error and fscking
   the index, which is likely better than just returning failure.
 * view_sync_set_log_view_range()'s caller handles 0 by rebuilding the
   missing changes in memory. So returning 0 makes it work better.
 * dsync_log_set() and mailbox_get_expunges_init() have fallbacks for
   handling 0, while -1 would just return an error.