Bug 29139: Add exceptions to relation accessors

Bug 29139: Add exceptions to relation accessors

We already had exceptions on the many-to-many links, but we didn't have
them for the middle table. The underlying dbic relations make it clear
which id's are being used for linking.  A 'credit' has 'credit_offsets',
a 'debit' has 'debit_offsets'.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>