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/serializer/serializer_stream.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var ReadableStream = require('stream').Readable,
+    inherits = require('util').inherits,
+    Serializer = require('./index');
+
+var SerializerStream = module.exports = function (node, options) {
+    ReadableStream.call(this);
+
+    this.serializer = new Serializer(node, options);
+
+    Object.defineProperty(this.serializer, 'html', {
+        //NOTE: To make `+=` concat operator work properly we define
+        //getter which always returns empty string
+        get: function () {
+            return '';
+        },
+        set: this.push.bind(this)
+    });
+};
+
+inherits(SerializerStream, ReadableStream);
+
+//Readable stream implementation
+SerializerStream.prototype._read = function () {
+    this.serializer.serialize();
+    this.push(null);
+};