Rescue keys file errors for 'list' command

We'd like a pretty message when using the `list` command and the file does exists. We already do this for `add` and `remove`, so we'll use the same rescue blocks here.

We'll now see:

```bash
$ gh-auth list --path=~/Desktop/does-not-exist
Keys file does not exist!

Create one now and try again:

  $ touch /Users/chris/Desktop/does-not-exist
```

Instead of:

```bash
$ gh-auth list --path=~/Desktop/does-not-exist
No such file or directory - /Users/chris/Desktop/does-not-exist 
(Github::Auth::KeysFile::FileDoesNotExistError)
```