added eslint await error

[ejahnGithub]

add the eslint to check the async func will return value after all the async calls finish. Also, test the eslint by removing await in below code block, and it would detect it.

    try {
      if (fs.existsSync(filePath)) {
        await targetInfo.verify(fs.createReadStream(filePath));
        return filePath;

ref to this PR: #362

[bdehamer]

This is great. Thanks for wiring this up!

In both the instances where you disabled the rule, I think the linter is showing us legit errors in our code (though no where near as serious as our previous missing `await)