Bw auths don't count circuit failures in descriptor mode

When we are using descriptor bandwidth (ie no feedback), we are unable to properly use circuit failure statistics to penalize nodes that are either attempting path bias, or are just experiencing CPU overload.

The fix should be simple. I think we just need to add another clause in aggregate.py where we check for use_circ_fails to also check for use_desc_bw and properly combine the pid_error and circ_error for that case (perhaps just by multiplying them).