Switch tests to mocha.

lib/mbtiles.js

@@ -164,7 +164,7 @@ MBTiles.prototype.getTile = function(z, x, y, callback) {
         'zoom_level = ? AND tile_column = ? AND tile_row = ?',
         z, x, y,
         function(err, row) {
-            if (!row || (err && err.errno == 1)) {
+            if ((!err && !row) || (err && err.errno == 1)) {
                 return callback(new Error('Tile does not exist'));
             } else if (err) {
                 return callback(err);