cp -n should not produce errors if file exists

``` sh
$> ls
fileA  fileB
$> shx cp -n fileA fileB
cp: dest file already exists: fileB
$> echo $?
130
$> cp -n fileA fileB # no output
$> echo $?
0 # no error
```

This is annoying when `shx cp -n` is used in a npm script as the script will then stop.
