Better error when relative path to generator does not exist

[JoshuaKGoldberg]

I'm trying to run yo on local repositories without going through npm link and the like. This actually works fine if you pass a path to yo that does exist:

git clone https://github.com/yeoman/generator-generator
yo ./generator-generator/app/index.js

# (after deprecation warnings)
? Your generator name (generator-yotest) 

But if a relative path that doesn't exist is passed, the error message is misleading:

yo ./does/not/exist.js

Error ./does/not/exist.js 

You don't seem to have a generator with the name “undefined” installed.
But help is on the way:

You can see available generators via npm search yeoman-generator or via http://yeoman.io/generators/. 
Install them with npm install undefined.

To see all your installed generators run yo --generators. Adding the --help option will also show subgenerators. 

If yo cannot find the generator, run yo doctor to troubleshoot your system.

Proposal: let's have yo give a specialized error message for a relative path that does not exist? Maybe:

Error ./does/not/exist.js 

The local generator "./does/not/exist.js" does not exist.

Double-check that that local path exists. If yo still cannot find the generator, run yo doctor to troubleshoot your system.