Return BadRequest instead of UnprocessableEntity for volumes API

Changes exception from UnprocessableEntity to BadRequest when
input validation fails. Also raise BadRequest instead of a nova
exception InvalidParameterValue within the SnapshotController
create method. This is some preparation work to make sharing
unittests between V2.1 and V2 easier.

Partially implements blueprint v2-on-v3-api

Change-Id: I5c5b6f9680378d51f7fea054b52b99cd6cbe54b0
Merge "Return BadRequest instead of UnprocessableEntity for volumes API"