STC fixes

- NPE when a switch has already been removed and an attempt is made
  to access stats on the switch.

- NPE in group provider if the system is in the process of shutting down.

- Lower ERROR to WARN when switch resources are not available.

Change-Id: I5b90ff328b19c2d661fde3a24e90614999c4c900
diff --git a/core/net/src/main/java/org/onosproject/net/resource/impl/ResourceDeviceListener.java b/core/net/src/main/java/org/onosproject/net/resource/impl/ResourceDeviceListener.java
index 1cd47aa..c9d4513 100644
--- a/core/net/src/main/java/org/onosproject/net/resource/impl/ResourceDeviceListener.java
+++ b/core/net/src/main/java/org/onosproject/net/resource/impl/ResourceDeviceListener.java
@@ -159,7 +159,7 @@
                 .map(adminService::register)
                 .ifPresent(success -> {
                    if (!success) {
-                       log.error("Failed to register Bandwidth for {}", portPath.id());
+                       log.warn("Failed to register Bandwidth for {}", portPath.id());
                    }
                 });