Update quota-class-set and quota-set thrown 500 error

[huangtianhua]

When update quota-class-set without the parameter "quota_class_set",the server throws a 500 error:
 "The server has either erred or is incapable of performing the requested operation."

Because the server doesn't check whether the parameter "quota_class_set" is in request body,the KeyError is not catched.

   for key in body['quota_class_set'].keys():
       ......

I think we should catch the KeyError and transfer the KeyError to 400(HTTPBadRequest) instead of 500.
Update quota-set has the same problem.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/55633

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/55633
Committed: http://github.com/openstack/nova/commit/cf6bf1c88687da5c18487ddfa2434a3943129847
Submitter: Jenkins
Branch: master

commit cf6bf1c88687da5c18487ddfa2434a3943129847
Author: huangtianhua <email address hidden>
Date: Fri Nov 8 16:05:17 2013 +0800

    Update quota-class-set/quota-set throw 500 error

    The server doesn't check whether the parameter "quota_class_set" is in request
    body.So the 500 error has been thrown.

    We should catch the KeyError and transfer the KeyError to
    400(HTTPBadRequest) instead of 500.

    Change-Id: I431a836bd35f846790497d6179515e8d559e2cd0
    Closes-Bug: #1249222