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/is-symbol/index.js

@@ -0,0 +1,35 @@
+'use strict';
+
+var toStr = Object.prototype.toString;
+var hasSymbols = require('has-symbols')();
+
+if (hasSymbols) {
+	var symToStr = Symbol.prototype.toString;
+	var symStringRegex = /^Symbol\(.*\)$/;
+	var isSymbolObject = function isRealSymbolObject(value) {
+		if (typeof value.valueOf() !== 'symbol') {
+			return false;
+		}
+		return symStringRegex.test(symToStr.call(value));
+	};
+
+	module.exports = function isSymbol(value) {
+		if (typeof value === 'symbol') {
+			return true;
+		}
+		if (toStr.call(value) !== '[object Symbol]') {
+			return false;
+		}
+		try {
+			return isSymbolObject(value);
+		} catch (e) {
+			return false;
+		}
+	};
+} else {
+
+	module.exports = function isSymbol(value) {
+		// this environment does not support Symbols.
+		return false && value;
+	};
+}