Fix NPE in FlowObjectiveManager when unregistering pipeconf drivers

The DriverManager was returning null instead of generating an
ItemNotFound exception as expected.

Change-Id: I4da6e07bfc3793ac0184cae40ce7f39b398e97f1
(cherry picked from commit 363660d96196c6da3b39b817bc7a8cb43df19330)
diff --git a/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java b/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java
index 8990822..31032b3 100644
--- a/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java
+++ b/core/net/src/main/java/org/onosproject/net/driver/impl/DriverManager.java
@@ -121,9 +121,12 @@
 
         // Special processing for devices with pipeconf.
         if (pipeconfService.ofDevice(deviceId).isPresent()) {
-            // No fallback for pipeconf merged drivers. Returns null if driver
-            // does not exist.
-            return getPipeconfMergedDriver(deviceId);
+            // No fallback for pipeconf merged drivers.
+            // Throws exception if pipeconf driver does not exist.
+            return nullIsNotFound(
+                    getPipeconfMergedDriver(deviceId),
+                    "Device is pipeconf-capable but a " +
+                            "pipeconf-merged driver was not found");
         }
 
         // Primary source of driver configuration is the network config.