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/path-is-absolute/index.js

@@ -0,0 +1,20 @@
+'use strict';
+
+function posix(path) {
+	return path.charAt(0) === '/';
+}
+
+function win32(path) {
+	// https://github.com/nodejs/node/blob/b3fcc245fb25539909ef1d5eaa01dbf92e168633/lib/path.js#L56
+	var splitDeviceRe = /^([a-zA-Z]:|[\\\/]{2}[^\\\/]+[\\\/]+[^\\\/]+)?([\\\/])?([\s\S]*?)$/;
+	var result = splitDeviceRe.exec(path);
+	var device = result[1] || '';
+	var isUnc = Boolean(device && device.charAt(1) !== ':');
+
+	// UNC paths are always absolute
+	return Boolean(result[2] || isUnc);
+}
+
+module.exports = process.platform === 'win32' ? win32 : posix;
+module.exports.posix = posix;
+module.exports.win32 = win32;