Description of problem:
When providing an invalid download_policy value, the error message returned is:
"displayMessage": "Validation failed: Download policy is not included in the list"

Version-Release number of selected component (if applicable):
Satellite 6.2 Beta Snap 4

How reproducible:
Always

Steps to Reproduce:
1. Create or enable a repository
2. submit a put request to /katello/api/repositories/<repo id>
   with an invalid value for download_policy

Actual results:
{
  "displayMessage": "Validation failed: Download policy is not included in the list",
  "errors": {
    "download_policy": [
      "is not included in the list"
    ]
  }
}

Expected results:
{
  "displayMessage": "Validation failed: Download policy is not included in the list: ['immediate', 'download_policy', 'background']",
  "errors": {
    "download_policy": [
      "is not included in the list: ['immediate', 'download_policy', 'background']"
    ]
  }
}