Rescue from invalid GDS API URLs
Rescue from invalid GDS API URLs

This adds exception handling for invalid GDS API URLs returning
422 Unprocessable Entity instead of 500 Internal Server Error.

An example of when this can happen:

```
GET /search/advanced?group=services'&topic=%2Fmoney%2Fbusiness-tax"

{"method":"GET","path":"/search/advanced","format":"html","controller":"advanced_search_finder","action":"show","status":200,"duration":0.86,"ip":"127.0.0.1","route":"advanced_search_finder#show","request_id":"2c2e27bf-3744-42f5-8805-0b9250b78d29","request":"GET /search/advanced?group=services%27\u0026topic=%2Fmoney%2Fbusiness-tax%22 HTTP/1.1","govuk_request_id":null,"varnish_id":null,"govuk_app_config":"1.10.0","source":"unknown","tags":["request"],"@timestamp":"2018-11-15T12:59:24.212Z","@version":"1"}
```

alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend