[JAVA-258]: prevent getLastError from being called in toString()

Author: agirbal

src/main/com/mongodb/WriteResult.java

@@ -138,7 +138,10 @@ public boolean isLazy(){
 
     @Override
     public String toString(){
-        return getLastError().toString();
+        CommandResult res = getCachedLastError();
+        if (res != null)
+            return res.toString();
+        return "N/A";
     }
 
     private long _lastCall;