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/atob/bower.json

@@ -0,0 +1,24 @@
+{
+  "name": "atob",
+  "description": "atob for isomorphic environments",
+  "main": "browser-atob.js",
+  "authors": [
+    "AJ ONeal <coolaj86@gmail.com> (https://coolaj86.com)"
+  ],
+  "license": "(MIT OR Apache-2.0)",
+  "keywords": [
+    "atob",
+    "browser"
+  ],
+  "homepage": "https://github.com/node-browser-compat/atob",
+  "moduleType": [
+    "globals"
+  ],
+  "ignore": [
+    "**/.*",
+    "node_modules",
+    "bower_components",
+    "test",
+    "tests"
+  ]
+}