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/debug/src/index.js

@@ -0,0 +1,10 @@
+/**
+ * Detect Electron renderer process, which is node, but we should
+ * treat as a browser.
+ */
+
+if (typeof process !== 'undefined' && process.type === 'renderer') {
+  module.exports = require('./browser.js');
+} else {
+  module.exports = require('./node.js');
+}