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/shebang-command/index.js

@@ -0,0 +1,19 @@
+'use strict';
+var shebangRegex = require('shebang-regex');
+
+module.exports = function (str) {
+	var match = str.match(shebangRegex);
+
+	if (!match) {
+		return null;
+	}
+
+	var arr = match[0].replace(/#! ?/, '').split(' ');
+	var bin = arr[0].split('/').pop();
+	var arg = arr[1];
+
+	return (bin === 'env' ?
+		arg :
+		bin + (arg ? ' ' + arg : '')
+	);
+};