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/lodash/_apply.js

@@ -0,0 +1,21 @@
+/**
+ * A faster alternative to `Function#apply`, this function invokes `func`
+ * with the `this` binding of `thisArg` and the arguments of `args`.
+ *
+ * @private
+ * @param {Function} func The function to invoke.
+ * @param {*} thisArg The `this` binding of `func`.
+ * @param {Array} args The arguments to invoke `func` with.
+ * @returns {*} Returns the result of `func`.
+ */
+function apply(func, thisArg, args) {
+  switch (args.length) {
+    case 0: return func.call(thisArg);
+    case 1: return func.call(thisArg, args[0]);
+    case 2: return func.call(thisArg, args[0], args[1]);
+    case 3: return func.call(thisArg, args[0], args[1], args[2]);
+  }
+  return func.apply(thisArg, args);
+}
+
+module.exports = apply;