Decorator content does too many different actions, so
let's split it into 3 decorators: @handle_errors,
@validate and @serialize. We can skip calling @serialize
in many places, because those methods just raise some
http error.
Also I've changed return value of all handlers from
serialized json (string) to python's structures, lists
and dicts. So It's become possible to implement different
serializers and I've added support for YAML output.
Change-Id: Ia3da3bd809bcca923d53666eca54def78c995f65
Closes-Bug: #1606211
6e31938a0f