Output strerror from xo_warn

Phabric keeps dropping my diff submissions so I'm putting this here.

Right now xo_warn() causes problems because it doesn't output the error number any longer.

This is what xo_warn should output:
```
/usr/src/lib/libxo % wc /nonexist
wc: /nonexist: open: No such file or directory
```

However libxo is broken and only outputs this:
```
/usr/src/lib/libxo % wc /nonexist         
wc: /nonexist: open
```