Optionally redirect critical or error reports to separate syslog facility.

By setting the `error_facility` property in the application environment reports with severity critical or error will be redirected to the specified facility. When not set, the current behavior is not affected.

Since having separate log files for informational and error messages is already a well-known feature for users of basho's (heavyweight) [lager](https://github.com/basho/lager) framework adopting it could make migration to `sasl_syslog` more easy.