Fix error status code for cloudpipe_update

When passing bad body in a request, most APIs return BadRequest response.
However, cloudpipe_update API doesn't do it.
This patch fixes the error status code and adds a unit test related to
this change.

Change-Id: I16a48e6913f52baa870e04196529fda619c03098
Merge "Fix error status code for cloudpipe_update"