Revert error behaviour for estimatedFeatureCount

Keep current production behavior of ignoreing errors when computing this stat and returning -1.
This is done as to no introduce any instability in production at the moment.

lib/cartodb/backends/layer-stats/mapnik-layer-stats.js

@@ -34,11 +34,16 @@ MapnikLayerStats.prototype.is = function (type) {
     return this._types[type] ? this._types[type] : false;
 };
 
-function queryPromise(dbConnection, query, adaptResults) {
+function queryPromise(dbConnection, query, adaptResults, errorHandler) {
     return new Promise(function(resolve, reject) {
         dbConnection.query(query, function (err, res) {
             if (err) {
-                reject(err);
+                if (errorHandler) {
+                    errorHandler(err);
+                }
+                else {
+                    reject(err);
+                }
             }
             else {
                 resolve(adaptResults(res));
@@ -71,11 +76,11 @@ function _getSQL(ctx, query, type='pre', zoom=0) {
 }
 
 function _estimatedFeatureCount(ctx) {
-    // TODO: restore -1 on errors behavior?
     return queryPromise(
         ctx.dbConnection,
         _getSQL(ctx, queryUtils.getQueryRowEstimation),
-        res => ({ estimatedFeatureCount: res.rows[0].rows })
+        res => ({ estimatedFeatureCount: res.rows[0].rows }),
+        () => ({ estimatedFeatureCount: -1 })
     );
 }