hostname: re-read file later when failed to update file

Previously, even when writing e.g. /etc/hostname fails, the static
hostname in Context is not restored. So, the subsequent call of the same
method succeeds:
```
$ sudo chattr +i /etc/hostname
$ sudo hostnamectl --static set-hostname aaa
Could not set static hostname: Access denied
$ echo $?
1
$ sudo hostnamectl --static set-hostname aaa
$ echo $?
0
```

This makes when updating file is failed, the saved stat is cleared. So,
the static hostname or machine information in the context are always
consistent to the corresponding files.