Bug 21702: (follow-up) Simplify checks and use standard fallback behaviour

Bug 21702: (follow-up) Simplify checks and use standard fallback behaviour

This patch acknowledges the fact that some checks here are done in a way
that is not standard these days. This checks are originally done
multiple times in different ways. output_and_exit_if_error should be
used.

This implies a minor behaviour change: if the passed (in the URL)
borrowernumber doesn't exist, it sends the user to a 'Patron doesn't
exist page' instead of the circulation page for the borrowernumber.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Isherwood <andrew.isherwood@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>