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/_Map.js

@@ -0,0 +1,7 @@
+var getNative = require('./_getNative'),
+    root = require('./_root');
+
+/* Built-in method references that are verified to be native. */
+var Map = getNative(root, 'Map');
+
+module.exports = Map;