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/graceful-fs/clone.js

@@ -0,0 +1,19 @@
+'use strict'
+
+module.exports = clone
+
+function clone (obj) {
+  if (obj === null || typeof obj !== 'object')
+    return obj
+
+  if (obj instanceof Object)
+    var copy = { __proto__: obj.__proto__ }
+  else
+    var copy = Object.create(null)
+
+  Object.getOwnPropertyNames(obj).forEach(function (key) {
+    Object.defineProperty(copy, key, Object.getOwnPropertyDescriptor(obj, key))
+  })
+
+  return copy
+}