Add an error message if `site-set` command can't create temp file

If `drush site-set` can't create the temp file it uses to store the DRUPAL_SITE variable, it fails silently, which isn't very helpful for the user. This adds an error message.

Before:
```
$ drush site-set @d8.dev
$
```

After:
```
$ drush site-set @d8.dev
Directory /tmp/drush-env exists, but is not writable. Please check directory permissions. [error]
Directory /tmp/drush-env exists, but is not writable. Please check directory permissions. [error]
$
```