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/acorn/bin/run_test262.js

@@ -0,0 +1,21 @@
+const fs = require("fs")
+const path = require("path")
+const run = require("test262-parser-runner")
+const parse = require("..").parse
+
+const unsupportedFeatures = [
+  "BigInt",
+  "class-fields",
+  "class-fields-private",
+  "class-fields-public",
+  "numeric-separator-literal"
+];
+
+run(
+  (content, {sourceType}) => parse(content, {sourceType, ecmaVersion: 10}),
+  {
+    testsDirectory: path.dirname(require.resolve("test262/package.json")),
+    skip: test => (test.attrs.features && unsupportedFeatures.some(f => test.attrs.features.includes(f))),
+    whitelist: fs.readFileSync("./bin/test262.whitelist", "utf8").split("\n").filter(v => v)
+  }
+)