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/_getNative.js

@@ -0,0 +1,17 @@
+var baseIsNative = require('./_baseIsNative'),
+    getValue = require('./_getValue');
+
+/**
+ * Gets the native function at `key` of `object`.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @param {string} key The key of the method to get.
+ * @returns {*} Returns the function if it's native, else `undefined`.
+ */
+function getNative(object, key) {
+  var value = getValue(object, key);
+  return baseIsNative(value) ? value : undefined;
+}
+
+module.exports = getNative;