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/_baseFor.js

@@ -0,0 +1,16 @@
+var createBaseFor = require('./_createBaseFor');
+
+/**
+ * The base implementation of `baseForOwn` which iterates over `object`
+ * properties returned by `keysFunc` and invokes `iteratee` for each property.
+ * Iteratee functions may exit iteration early by explicitly returning `false`.
+ *
+ * @private
+ * @param {Object} object The object to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @param {Function} keysFunc The function to get the keys of `object`.
+ * @returns {Object} Returns `object`.
+ */
+var baseFor = createBaseFor();
+
+module.exports = baseFor;