Bug 35394: Correction to booked items logic

Bug 35394: Correction to booked items logic

This patch fixes the overly extreme message that was designed to appeared
when someone tried to check out a booked item to the wrong user.
However, the message never appeared due to a logic error within
CanBookBeIssued which this patch also resolved.

Test plan
1) Make an item bookable
2) Add a booking for the item to patron A from tomorrow
3) In the database, update the booking start_date to today (or wait a
   day before the next step)
4) Attempt to check the book out to patron B.
   Note that you are displayed with the new message 'The item is booked
   for another patron' and you are unable to check the item out.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>