If we try to add already existing exchange then return id of that exchange in the error message

[sgerodes]

Right now the error message is { 'api_key': ['This account is already connected!'] }

Add smth like this: { 'api_key': [ 'This account is already connected!' ] 'account_id': 123456 }

Its easy on your side, but would be more tedious on our side to search for it.