Responding with 404 when updating an immutable attribute of a non-existing resource

[davidsarosap]

Responding with 404 when updating an immutable attribute of a non-exi… (#235)

…sting resource

Currently when doing a Patch of an immutable attribute of a non-existing
resource the status code is 400.
But it should be a 404, because the validation of the body should happen
after the validation of the path.

Summary:
PATCH /Groups/nonexisting

{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:PatchOp"
  ],
  "Operations": [
    {
      "op": "remove",
      "path": "displayName",
      "value": "aName"
    }
  ]
}

Current response:

{
  "status": "400",
  "scimType": "invalidValue",
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:Error"
  ],
  "detail": "Attribute with name urn:ietf:params:scim:schemas:core:2.0:Group:displayName is required and cannot not be removed"
}

After patch applied:

{
  "status": "404",
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:Error"
  ],
  "detail": "HTTP 404 Not Found"
}