Detect node store batch write failures (RIPD-270):

* Raise open file limit from the soft max to the hard max.

src/ripple/nodestore/backend/HyperDBFactory.cpp

@@ -172,7 +172,10 @@ public:
 
         hyperleveldb::WriteOptions const options;
 
-        m_db->Write (options, &wb).ok ();
+        auto ret = m_db->Write (options, &wb);
+
+        if (!ret.ok ())
+            throw std::runtime_error ("storeBatch failed: " + ret.ToString());
     }
 
     void