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/_copySymbols.js

@@ -0,0 +1,16 @@
+var copyObject = require('./_copyObject'),
+    getSymbols = require('./_getSymbols');
+
+/**
+ * Copies own symbols of `source` to `object`.
+ *
+ * @private
+ * @param {Object} source The object to copy symbols from.
+ * @param {Object} [object={}] The object to copy symbols to.
+ * @returns {Object} Returns `object`.
+ */
+function copySymbols(source, object) {
+  return copyObject(source, getSymbols(source), object);
+}
+
+module.exports = copySymbols;