Add error code 2601

[RasmusN]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Adds error code 2601 to be represented as gorm.ErrDuplicatedKey in the error translator.

User Case Description

SQL Server have two slightly different error codes for duplicate key errors: 2627 occurs on unique/PK constraint violations, while 2601 is triggered by unique index violations. Both indicate duplicate keys but stem from different schema definitions.

Description from sqlserver

select * from sysmessages where (error = 2627 or error = 2601) and msglangid = 1033

error	severity	dlevel	description	msglangid
2601	14	0	Cannot insert duplicate key row in object '%.*ls' with unique index '%.*ls'. The duplicate key value is %ls.	1033
2627	14	0	Violation of %ls constraint '%.*ls'. Cannot insert duplicate key in object '%.*ls'. The duplicate key value is %ls.	1033