improves conditional update error message to include extent (apache#4214)

Author: keith-turner

server/base/src/main/java/org/apache/accumulo/server/metadata/ConditionalTabletsMutatorImpl.java

@@ -83,7 +83,7 @@ public Ample.OperationRequirements mutateTablet(KeyExtent extent) {
     }
 
     Preconditions.checkState(extents.putIfAbsent(extent.toMetaRow(), extent) == null,
-        "Duplicate extents not handled");
+        "Duplicate extents not handled %s", extent);
     return new ConditionalTabletMutatorImpl(this, context, extent, mutations::add,
         rejectedHandlers::put);
   }