base: fix CurrencyConstraint

It was using "Reference" instead of the dedicated term "Currency Code".

Attribute Unicity constraint message was not correctly including the
reference, the error message was always "Another currency with reference
None already exists".
22 jobs for fix/currency_contraint in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:02:44

failed ERP5.UnitTest-Master

02:45:02

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:06

failed Wendelin.UnitTest-Master

00:40:18

passed ERP5.CodingStyleTest-Master

00:49:09

passed ERP5.CodingStyleTest-Master

00:59:06

passed ERP5.CodingStyleTest-Master

00:42:02

failed ERP5.UnitTest-Master

01:23:58

failed ERP5.UnitTest-Master

01:55:33

failed ERP5.UnitTest-Master

02:05:30

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:27

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:52

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:24

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:49

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:59

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:15

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:32

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:19

failed Wendelin.UnitTest-Master

00:58:14

passed Wendelin.UnitTest-Master

00:32:05

passed Wendelin.UnitTest-Master

00:41:04