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/_coreJsData.js

@@ -0,0 +1,6 @@
+var root = require('./_root');
+
+/** Used to detect overreaching core-js shims. */
+var coreJsData = root['__core-js_shared__'];
+
+module.exports = coreJsData;