Add PreconditionFailed exception for HTTP 412 errors

This will be an expected error code from Neutron API tests
when testing the compare-and-swap update capabilities being
added in I7d97d6044378eb59cb2c7bdc788dc6c174783299.

Otherwise the tests need to catch UnexpectedResponseCode and
extract the error number from the body, which looks ugly.

Change-Id: Ic2576438228ea1e15de01779735f7c3ca048c4e5
3 files changed