Validate: Can't use unix pipes or return code

```
$ github-codeowners validate
Found rules which did not match any files [
  '/some_file.yml @team'
]
$ echo $?
0
```

I would have expected this to return an error.

Further I tried to work around this by using `wc -l` but no dice:

```
$ github-codeowners validate | wc -l
Found rules which did not match any files [
  '/some_file.yml @team'
]
       0
```