If post non valid data to `/ws/2/tag` it return 502 (Bad Gateway) error html page, instead of 400 (Bad Request) or something like it

Example of data
```
curl -X POST --digest -u 'login:password' -H'Content-Type: application/xml; charset=utf-8' --data 'random trash' https://musicbrainz.org/ws/2/tag?client=some.app-0.0.0
```