Server error (500) returned after editing Catalog item with used name

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`Services > Catalogs > Catalogs > Configuration > New/Edit`

**Description**
- When you add a new Catalog with used name it returns 400 error
- However when you edit some Catalog and change its name to used name **it returns 500 error**

`http://localhost:3000/api/service_catalogs/45`

**body**

```
{
"action": "edit",
"resource": {
"name": "Aa" // something what is already used
}
}
```

**response**

500 Interval Server Error

```
{
"error": {
"kind": "internal_server_error",
"message": "Validation failed: ServiceTemplateCatalog: Name has already been taken",
"klass": "ActiveRecord::RecordInvalid"
}
}
```

Start at the `/api/service_catalogs/45` edit endpoint and reproduce the request using a name already used by another Catalog. Trace why duplicate-name validation returns 500 for edits while creation returns 400; done means the edit responds with the appropriate 400 validation error instead of an internal server error.

Written by the indexing model from the issue text.