fixed crash where `err` was not defined in proxy error listener.

```
UNCAUGHT EXCEPTION!
Server error: ReferenceError: err is not defined
    at AuditProxy.<anonymous> (/Users/mandric/dev/medic-api/server.js:50:17)
    at AuditProxy.emit (events.js:95:17)
    at PassThroughExt.<anonymous> (/Users/mandric/dev/medic-api/audit-proxy.js:78:12)
    at PassThroughExt.emit (events.js:117:20)
    at done (/Users/mandric/dev/medic-api/node_modules/pass-stream/node_modules/readable-stream/lib/_stream_transform.js:191:19)
    at /Users/mandric/dev/medic-api/node_modules/pass-stream/node_modules/readable-stream/lib/_stream_transform.js:132:9
    at /Users/mandric/dev/medic-api/audit-proxy.js:63:18
    at /Users/mandric/dev/medic-api/node_modules/couchdb-audit/couchdb-audit/log.js:90:20
    at /Users/mandric/dev/medic-api/node_modules/couchdb-audit/node_modules/async/lib/async.js:254:17
    at done (/Users/mandric/dev/medic-api/node_modules/couchdb-audit/node_modules/async/lib/async.js:129:15)
```