DeviceManager: lowering harmless log level

Change-Id: I0913b0c7b352d4a53a72ee7cc8a4781da722973c
diff --git a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
index b15872b..546b497 100644
--- a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
@@ -186,7 +186,7 @@
         if (provider != null) {
             return provider.isReachable(deviceId);
         } else {
-            log.error("Provider not found for {}", deviceId);
+            log.debug("Provider not found for {}", deviceId);
             return false;
         }
     }