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/_castFunction.js

@@ -0,0 +1,14 @@
+var identity = require('./identity');
+
+/**
+ * Casts `value` to `identity` if it's not a function.
+ *
+ * @private
+ * @param {*} value The value to inspect.
+ * @returns {Function} Returns cast function.
+ */
+function castFunction(value) {
+  return typeof value == 'function' ? value : identity;
+}
+
+module.exports = castFunction;