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/which-module/index.js

@@ -0,0 +1,9 @@
+'use strict'
+
+module.exports = function whichModule (exported) {
+  for (var i = 0, files = Object.keys(require.cache), mod; i < files.length; i++) {
+    mod = require.cache[files[i]]
+    if (mod.exports === exported) return mod
+  }
+  return null
+}