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/_mapCacheClear.js

@@ -0,0 +1,21 @@
+var Hash = require('./_Hash'),
+    ListCache = require('./_ListCache'),
+    Map = require('./_Map');
+
+/**
+ * Removes all key-value entries from the map.
+ *
+ * @private
+ * @name clear
+ * @memberOf MapCache
+ */
+function mapCacheClear() {
+  this.size = 0;
+  this.__data__ = {
+    'hash': new Hash,
+    'map': new (Map || ListCache),
+    'string': new Hash
+  };
+}
+
+module.exports = mapCacheClear;