Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/isarray/index.js

@@ -0,0 +1,5 @@
+var toString = {}.toString;
+
+module.exports = Array.isArray || function (arr) {
+  return toString.call(arr) == '[object Array]';
+};