Rackspace: Don't raise ResourceFailure exceptions

ResourceFailure is meant to be a wrapper around other exceptions that's
invoked by top-level resource operations; don't use it as an all-purpose
exception.

Change-Id: Ie15cad32ee31beca9b0f7e9a9473ced9956a2e3b
Merge "Rackspace: Don't raise ResourceFailure exceptions"