Retry handling is broken since the gRPC changes (wrong error classification).
Will come back at some point, hopefully by merging the replication
driver retry infrastructure.
However, the simpler architecture allows an easy fix for the problem
that the pruner practically gave up on the first error it encountered.
fixes#123
Pruner now backs off as soon as there is an error, making that error the
Error field in the pruner report.
The error is also stored in the specific *fs that failed, and we
maintain an error counter per *fs to de-prioritize those fs that failed.
Like with replication, the de-prioritization on errors is to avoid '
getting stuck' with an individual filesystem until the watchdog hits.
