Suppress exception stack traces on warning conditions to prevent STC faiures for non-fatal situations

Change-Id: I10d4945e92b3674f00725597d00fc13cc140c943
diff --git a/protocols/openflow/api/src/main/java/org/onosproject/openflow/controller/driver/AbstractOpenFlowSwitch.java b/protocols/openflow/api/src/main/java/org/onosproject/openflow/controller/driver/AbstractOpenFlowSwitch.java
index 51875b0..0ba3bd9 100644
--- a/protocols/openflow/api/src/main/java/org/onosproject/openflow/controller/driver/AbstractOpenFlowSwitch.java
+++ b/protocols/openflow/api/src/main/java/org/onosproject/openflow/controller/driver/AbstractOpenFlowSwitch.java
@@ -287,7 +287,7 @@
                 }
                 this.agent.processMessage(dpid, m);
             } catch (Exception e) {
-                log.warn("Unhandled exception processing {}@{}", m, dpid, e);
+                log.warn("Unhandled exception processing {}@{}:{}", m, dpid, e.getMessage());
             }
         } else {
             log.trace("Dropping received message {}, was not MASTER", m);