Fixes #6396: an error removing a DHCP record (record doesn't exist) stop...

...s the delete process for a host
Hmm, would a 404 here not cause much the same issue? The foreman code parses proxy responses with this:

```
if response and response.code >= 200 and response.code < 300
```

So a 404 will still trigger the same result as the catchall 400 below.