sievec prints an error if no dovecot running with mail_plugins = stats

[SATOH Fumiyasu]

Dovecot 2.2.36 + Pigeonhole 0.4.24

Is this an intended behavior?

```
# service dovecot status
dovecot is stopped
# doveconf mail_plugins
mail_plugins =  stats
# ls foo.*
foo.sieve
# sievec foo.sieve
sievec(root): Error: stats: open(/var/run/dovecot/stats-mail) failed: No such device or address
# echo $?
0
# ls foo.*
foo.sieve  foo.svbin
```

-- 
-- Name: SATOH Fumiyasu @ OSS Technology Corp. (fumiyas @ osstech co jp)
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