Add #toString to EmptyConfiguredTarget and ValueOrException$ValueOrExceptionValueImpl to help diagnose bug.

PiperOrigin-RevId: 238919901
diff --git a/src/main/java/com/google/devtools/build/skyframe/ValueOrException.java b/src/main/java/com/google/devtools/build/skyframe/ValueOrException.java
index e26cb1d..11d467e 100644
--- a/src/main/java/com/google/devtools/build/skyframe/ValueOrException.java
+++ b/src/main/java/com/google/devtools/build/skyframe/ValueOrException.java
@@ -77,6 +77,11 @@
       return null;
     }
 
+    @Override
+    public String toString() {
+      return "ValueOrExceptionValueImpl:" + value;
+    }
+
     @SuppressWarnings("unchecked")
     static <E extends Exception> ValueOrExceptionValueImpl<E> ofNullValue() {
       return (ValueOrExceptionValueImpl<E>) NULL;