datastore/request: add API error handling

lib/common/util.js

@@ -149,7 +149,8 @@ function handleResp(err, resp, body, callback) {
     return;
   }
   if (resp && (resp.statusCode < 200 || resp.statusCode > 299)) {
-    callback(new Error('error during request, statusCode: ' + resp.statusCode));
+    callback(new Error('error during request, statusCode: ' + resp.statusCode +
+                       ', body: ' + body));
     return;
   }
   callback(null, body, resp);

lib/datastore/transaction.js

@@ -446,7 +446,13 @@ Transaction.prototype.makeReq = function(method, req, respType, callback) {
         buffer = Buffer.concat([buffer, chunk]);
       });
       resp.on('end', function() {
-        callback(null, respType.decode(buffer));
+        util.handleResp(null, resp, buffer.toString(), function(err) {
+          if (err) {
+            callback(err);
+            return;
+          }
+          return callback(null, respType.decode(buffer));        
+        });
       });
     });
     remoteStream.on('error', callback);