datastore/request: add API error handling

lib/datastore/transaction.js

@@ -446,7 +446,15 @@ Transaction.prototype.makeReq = function(method, req, respType, callback) {
         buffer = Buffer.concat([buffer, chunk]);
       });
       resp.on('end', function() {
-        callback(null, respType.decode(buffer));
+        if (resp.statusCode != 200) {
+          return callback({
+            error: 'request failed',
+            request: request,
+            response: resp,
+            body: buffer.toString()
+          }, null);
+        }
+        return callback(null, respType.decode(buffer));        
       });
     });
     remoteStream.on('error', callback);