add grid tests and return error with invalid grid

lib/mbtiles.js

@@ -243,8 +243,8 @@ MBTiles.prototype.grid = function(x, y, z, callback) {
                         return memo;
                     }, {});
                 callback(null, _(JSON.parse(grid)).extend({ data: data }));
-            } catch(err) {
-                callback(err);
+            } catch (err) {
+                callback('Grid is invalid');
             }
         }
     );