Demote Files.list() errors when distributor is enabled

Since Files.list() errors for the 'waiting' and 'running' dirs is common
and normal when running with a multi-primary setup, demote them to
"atFine()" level when the distributor is enabled. This avoids making the
logs extrememly noisy during normal multi-primary operation.

Change-Id: Iefc96fb79754dad9201afbc194d85194ef257198