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/_cloneSymbol.js

@@ -0,0 +1,18 @@
+var Symbol = require('./_Symbol');
+
+/** Used to convert symbols to primitives and strings. */
+var symbolProto = Symbol ? Symbol.prototype : undefined,
+    symbolValueOf = symbolProto ? symbolProto.valueOf : undefined;
+
+/**
+ * Creates a clone of the `symbol` object.
+ *
+ * @private
+ * @param {Object} symbol The symbol object to clone.
+ * @returns {Object} Returns the cloned symbol object.
+ */
+function cloneSymbol(symbol) {
+  return symbolValueOf ? Object(symbolValueOf.call(symbol)) : {};
+}
+
+module.exports = cloneSymbol;