`entry.String()` returns both the string representation of the log entry
and a possible error. Before this patch we were sending both to the log
writer, which accepts a varargs `interface{}` ending up with spurious
`<nil>` lines in syslog.

With this patch we try to get to the string representation and perform
an early return in case of errors, then pass only the string to the
underlying `service.Logger`.

Ref #2453