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/_LodashWrapper.js

@@ -0,0 +1,22 @@
+var baseCreate = require('./_baseCreate'),
+    baseLodash = require('./_baseLodash');
+
+/**
+ * The base constructor for creating `lodash` wrapper objects.
+ *
+ * @private
+ * @param {*} value The value to wrap.
+ * @param {boolean} [chainAll] Enable explicit method chain sequences.
+ */
+function LodashWrapper(value, chainAll) {
+  this.__wrapped__ = value;
+  this.__actions__ = [];
+  this.__chain__ = !!chainAll;
+  this.__index__ = 0;
+  this.__values__ = undefined;
+}
+
+LodashWrapper.prototype = baseCreate(baseLodash.prototype);
+LodashWrapper.prototype.constructor = LodashWrapper;
+
+module.exports = LodashWrapper;