Fix: external mergers exceptions (RTC-41300)

If there are two purchase orders/order despatch advices/goods receipts being processed by merger, that refer to same stores/suppliers/etc. and first one has thrown exception, second one is then processed properly anyway.
If goods receipt is received for order line and its item identifier does not match, ordered quantity of order line item is exported properly.

List of supported item units (RTC-39325)

Items imported from item service are saved in procurement database with unit (Norwegian code) from the list: stk, m, kg, l, dm3. If item has assigned unit not from this list, its unit is saved as 'stk' as default. Units of existing items are properly migrated with an upgrade script. Quantity of order line and when receiving goods cannot have decimals when item unit is 'stk' (replaced existing rule for unit 'Pcs').