No verification that input file exists (and obscure error message)

Just wanted to leave a not that the library does not check whether the file exists, and throws a difficult to interpret error. Dropping it here for so that is searchable.

```
obj2gltf -i this/file/does/not/exit.obj -o /foo/bar
/usr/local/lib/node_modules/obj2gltf/node_modules/universalify/index.js:15
}, 'name', { value: fn.name })
^

TypeError: Cannot read property 'name' of undefined
at exports.fromCallback (/usr/local/lib/node_modules/obj2gltf/node_modules/universalify/index.js:15:26)
at Object. (/usr/local/lib/node_modules/obj2gltf/node_modules/fs-extra/lib/fs/index.js:57:27)
at Module._compile (module.js:652:30)
at Object.Module._extensions..js (module.js:663:10)
at Module.load (module.js:565:32)
at tryModuleLoad (module.js:505:12)
at Function.Module._load (module.js:497:3)
at Module.require (module.js:596:17)
at require (internal/module.js:11:18)
at Object. (/usr/local/lib/node_modules/obj2gltf/node_modules/fs-extra/lib/index.js:5:6)
make: *** [obj2gltf_test2] Error 1
```

Start by reproducing the documented command with a nonexistent input path: `obj2gltf -i this/file/does/not/exit.obj -o /foo/bar`. Trace the input-path handling from the CLI entry point and make the failure identify the missing file clearly; done means the command no longer exposes the shown obscure stack trace.

Written by the indexing model from the issue text.