Maybe add new type of errors in the bandwidth file

As commented in https://trac.torproject.org/projects/tor/ticket/30905#comment:9

Things that are not really failures:

when loading the results, the relays that changed ip are excluded. I could add a patch to count all the results first, which reduces the number of failures in around 1000.
when sbws is restarted, all the queued relays to measure that didn't start to be measured yet, are not saved in the results, but the attempt has already been count. We could also add a patch to count the attempt in measure_relay, instead of main_loop, though then we won't know the number of attempts counting from the moment they were queued.
real failures would happen when result_putter_error is triggered, or some bug makes sbws stall, which then will print the TICKET_MSG error. In the 1st case, we could actually count an error.