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-key/index.js

@@ -0,0 +1,13 @@
+'use strict';
+module.exports = opts => {
+	opts = opts || {};
+
+	const env = opts.env || process.env;
+	const platform = opts.platform || process.platform;
+
+	if (platform !== 'win32') {
+		return 'PATH';
+	}
+
+	return Object.keys(env).find(x => x.toUpperCase() === 'PATH') || 'Path';
+};