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/@babel/parser/bin/babel-parser.js

@@ -0,0 +1,16 @@
+#!/usr/bin/env node
+/* eslint no-var: 0 */
+
+var parser = require("..");
+var fs = require("fs");
+
+var filename = process.argv[2];
+if (!filename) {
+  console.error("no filename specified");
+  process.exit(0);
+}
+
+var file = fs.readFileSync(filename, "utf8");
+var ast = parser.parse(file);
+
+console.log(JSON.stringify(ast, null, "  "));