Assume loan list hasn't changed when unable to communicate with distributor

[leonardr]

If there's a problem connecting to a distributor to sync a patron's loans list, we assume that the patron has no loans from that distributor. Instead, we should assume that the patron's loans have not changed.

If the loans are still available and have been downloaded already, this should allow the patron to read their books. If it's not possible to get the book because of the same distributor problem that prevents loan sync, at least it doesn't make the situation worse by removing the loan.

This will make a situation worse in some cases, as loans can stick around after they should have expired, but distributor APIs generally aren't nonfunctional for that long.