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/_getHolder.js

@@ -0,0 +1,13 @@
+/**
+ * Gets the argument placeholder value for `func`.
+ *
+ * @private
+ * @param {Function} func The function to inspect.
+ * @returns {*} Returns the placeholder value.
+ */
+function getHolder(func) {
+  var object = func;
+  return object.placeholder;
+}
+
+module.exports = getHolder;