Ignore HTML response

Sometimes an upstream may return an error with HTML. For example public upstreams.

Right now Dshakle produces the following log:
```
WARN | ResponseParser | Failed to parse JSON from upstream: Unexpected character ('<' (code 60)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')
at [Source: (byte[])"




Site Unavailable
....
```

Also it seems to break WS connection.

Need to catch such errors, and just mark upstream as unavailable until it's restored.