Monitor returns invalid response when REST API down

[steven-sheehy]

Detailed Description
During testnet upgrade of REST API, the new monitor started returning a stacktrace instead of a JSON response.

Actual Behavior
Steps to reproduce the behavior:

1. Stop REST API
2. Go to http://monitor/api/v1/status
3. Stacktrace in browser:

TypeError: Converting circular structure to JSON
    --> starting at object with constructor 'Object'
    |     property 'testResults' -> object with constructor 'Array'
    --- index 1 closes the circle
    at JSON.stringify (<anonymous>)
    at stringify (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/response.js:1123:12)
    at ServerResponse.json (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/response.js:260:14)
    at ServerResponse.send (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/response.js:158:21)
    at /opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/server.js:59:31
    at Layer.handle [as handle_request] (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/router/layer.js:95:5)
    at next (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/router/layer.js:95:5)
    at /opt/restapi/hedera-mirror-rest/monitoring/monitor_apis/node_modules/express/lib/router/index.js:281:22```

**Expected Behavior**
JSON output

**Environment:**
- Node: v12
- OS: Ubuntu 18.04
- Version: v1.5.1

**Additional Context**