unexpand: show error message if a directory is specified

GNU `unexpand` shows an error if the user specifies a directory and returns `1` as exit code:
```
$ echo "content" > file
$ unexpand dir file
unexpand: dir: Is a directory
content
$ echo $?
1
```
uutils `unexpand`, on the other hand, doesn't output anything and returns `0` as exit code:
```
$ echo "content" > file
$ cargo run unexpand dir file
$ echo $?
0
```