`@querystring-search` endpoint doesn't validate input sufficiently

[davisagli]

There are various ways to give @querystring-search invalid input that causes a 500 error. These should return a 400 Bad Request response instead, to avoid cluttering error logs.

Examples:

1. https://demo.plone.org/++api++/@querystring-search?query={ (Raises DeserializationError)
2. https://demo.plone.org/++api++/@querystring-search?query={"b_size":"1*100","query":[{"i":"portal_type","o":"plone.app.querystring.operation.selection.any","v":["News+Item"]}]} (raises ValueError)
3. https://demo.plone.org/++api++/@querystring-search?query={"query":[{"i":"portal_type","o":"bogus","v":["News+Item"]}]} (raises KeyError)