Log circuit errors at higher verbosity

All circuit reactor errors are currently logged at debug level (as of !3267 (merged)).

We might want to log some or all them at higher verbosity (for example, as log-ratelimited warnings), because some errors can be indicative of a bug in the circuit reactor (for example, half-streams are expired on a timer, so errors like UnknownStream can be incorrectly triggered if we expire streams too aggressively).