Manager: add more logging when worker cannot be marked as 'seen'

SQLite often errors out on this with only `interrupted (9)` as message.
This logging should at least tell us whether it's our own "background
context" timing out, or whether something else fishy is going on.

internal/manager/api_impl/workers.go

@@ -478,7 +478,14 @@ func (f *Flamenco) workerSeen(
 
 	err := f.persist.WorkerSeen(bgCtx, w)
 	if err != nil {
+		if bgCtx.Err() != nil {
+			logger.Error().
+				Err(err).
+				AnErr("contextError", bgCtx.Err()).
+				Msg("error marking Worker as 'seen' in the database, database operation timed out")
+		} else {
 			logger.Error().Err(err).Msg("error marking Worker as 'seen' in the database")
+		}
 		return err
 	}
 	return nil