API 'PUT' should return a 400 - Bad Request when an ForbiddenAttribute is encountered

When trying to PUT to a read-only attribute the following occurs:
    >>> url = '/firefox/trunk/0.9.2/+file/firefox-0.9.2.orig.tar.gz/file'
    >>> response = webservice.put(url, 'application/x-tar-gz', 'fakefiledata')
    >>> print response
    HTTP/1.1 500 Internal Server Error
    ...
    ForbiddenAttribute: ('libraryfile', <ProductReleaseFile...

The response should be 400 - Bad Request.

Related OOPSes: OOPS-982EA99, OOPS-982ED80, OOPS-982ED82, OOPS-982ED85