errors-only mode for `humility counters ipc`

[hawkw]

Bryan pointed out that it would be nice to have a way to surface any IPCs that have failed without having to wade through a bunch of stuff that's working fine. We could add a --errors flag or something and just show a list of those IPC methods and clients with non-zero error counts.

Could potentially do something like this for other counters, but error entries in a ringbuf aren't always Err variants the way the IPC counters are, so it might be less accurate...