setup: return the desc for failed setting on error

This commit improves the error printed when cli-supplied args
fail, adding at least the key name to what used to be just a
stringified errno:

```
$ src/rototiller --module=shapes,scale=99

Shape type:
 0:   circle
 1: pinwheel
 2:  rhombus
 3:     star
Enter a value 0-3 [1 (pinwheel)]:
Fatal error: unable to use args for setting "scale": Invalid argument
$
```

0 files changed, 0 insertions, 0 deletions