better logs

lib/server.js

@@ -1158,14 +1158,20 @@ WalletService.prototype._sampleFeeLevels = function(network, points, cb) {
       return cb(err);
     }
 
+    var failed = [];
     var levels = _.zipObject(_.map(points, function(p) {
       var feePerKb = _.isObject(result) ? +result[p] : -1;
-      if (feePerKb < 0) {
-        log.info('Could not compute fee estimation (nbBlocks=' + p + ')');
-      }
+      if (feePerKb < 0)
+        failed.push(p);
+
       return [p, Utils.strip(feePerKb * 1e8)];
     }));
 
+    if (failed.length) {
+      var logger = network == 'livenet' ? log.warn : log.debug;
+      logger('Could not compute fee estimation in ' + network + ': ' + failed.join(','));
+    }
+
     return cb(null, levels);
   });
 };