Previously when the upstream application (e.g. Unicorn) timed out,
Workhorse would report an error message of the form:

```json
{
  "correlation_id": "k4mKg0snXG3",
  "error": "badgateway: failed after 61s: EOF",
  "level": "error",
  "method": "GET",
  "msg": "error",
  "time": "2019-12-02T22:05:09-08:00",
  "uri": "/admin"
}
```

Since Sentry attempts to group similar events by some fingerprint that
includes the message, the changes in timings would trick Sentry into
creating a new event. To avoid that, the new log message now looks like:

```json
{
  "correlation_id": "k4mKg0snXG3",
  "duration_ms": 60599,
  "error": "badgateway: failed to receive response: EOF",
  "level": "error",
  "method": "GET",
  "msg": "error",
  "time": "2019-12-02T22:05:09-08:00",
  "uri": "/admin"
}
```

The inclusion of `duration_ms` and the standardized `error` message makes
it easier for users to filter and parse this error.