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/parse5/lib/sax/dev_null_stream.js

@@ -0,0 +1,14 @@
+'use strict';
+
+var WritableStream = require('stream').Writable,
+    util = require('util');
+
+var DevNullStream = module.exports = function () {
+    WritableStream.call(this);
+};
+
+util.inherits(DevNullStream, WritableStream);
+
+DevNullStream.prototype._write = function (chunk, encoding, cb) {
+    cb();
+};