Load holdings by id doesn't load when there are more than one credentials

Steps to reproduce:

1. Create 2 or more credentials (without assigned users)
2. Call endpoint POST /eholdings/loading/kb-credentials/<id>

Expected outcome: Holdings loading started if the user is assigned to the credentials

• if a user not assigned to the credentials he is trying to load the following error message should be returned:

  "KB Credentials do not exist or user with userId = %s  is not assigned to any available knowledgebase." 
  

  Actual outcome:

{
    "errors": [
        {
            "title": "User credentials not found: userId = e64be8a5-51f4-5a2b-92c5-0bcef12e767e"
        }
    ],
    "jsonapi": {
        "version": "1.0"
    }
}