toString for PartialFailureConstraint

- for better representation on CLI, etc.

Change-Id: I8575608832f7c97ee6123ddd8d217d02d35d3bb7
diff --git a/core/api/src/main/java/org/onosproject/net/intent/constraint/PartialFailureConstraint.java b/core/api/src/main/java/org/onosproject/net/intent/constraint/PartialFailureConstraint.java
index 5c00d06..ec43e08 100644
--- a/core/api/src/main/java/org/onosproject/net/intent/constraint/PartialFailureConstraint.java
+++ b/core/api/src/main/java/org/onosproject/net/intent/constraint/PartialFailureConstraint.java
@@ -48,4 +48,9 @@
         }
         return false;
     }
+
+    @Override
+    public String toString() {
+        return "PartialFailureConstraint";
+    }
 }