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/parser/plain_text_conversion_stream.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var ParserStream = require('./parser_stream'),
+    inherits = require('util').inherits,
+    $ = require('../common/html').TAG_NAMES;
+
+var PlainTextConversionStream = module.exports = function (options) {
+    ParserStream.call(this, options);
+
+    // NOTE: see https://html.spec.whatwg.org/#read-text
+    this.parser._insertFakeElement($.HTML);
+    this.parser._insertFakeElement($.HEAD);
+    this.parser.openElements.pop();
+    this.parser._insertFakeElement($.BODY);
+    this.parser._insertFakeElement($.PRE);
+    this.parser.treeAdapter.insertText(this.parser.openElements.current, '\n');
+    this.parser.switchToPlaintextParsing();
+};
+
+inherits(PlainTextConversionStream, ParserStream);