Nailgun validators should be unit-tested, not integration-tested

We already have two types of tests - integration and unit. But integration tests are outnumbering units dramatically. Here's what we can do with it:

- All validators should be ONLY unit-tested, with various input data and checking errors raised.
- There should be some kind of basic mapping of standard validation errors onto HTTP error codes, for example:
   * errors.AlreadyExists : HTTP 409
   * errors.InvalidData : HTTP 400
   * errors.CannotCreateObject: HTTP 400
and so on. This code should be located in basic handler and tested by integration tests
- All handlers which use validators raising errors which are not in the list of standard ones, should be tested on proper reaction on raising this error by passing sample invalid data in integration tests.
- All other tests and hacks on REST data validation may be removed, because we won't need them anymore.