Add response size and dev errors to user-events dashboard

## Description
* Add chart to display request response size
* Split the error dashboard into `System errors` and `Developer errors`.

## My changes affect the following components

- [ ] API
- [ ] Controller
- [ ] Message Bus (e.g., Kafka)
- [ ] Loadbalancer
- [ ] Invoker
- [ ] Intrinsic actions (e.g., sequences, conductors)
- [ ] Data stores (e.g., CouchDB)
- [ ] Tests
- [ ] Deployment
- [x] Monitoring
- [ ] CLI
- [ ] General tooling
- [ ] Documentation

## Types of changes

- [ ] Bug fix (generally a non-breaking change which closes an issue).
- [x] Enhancement or new feature (adds new functionality).
- [ ] Breaking change (a bug fix or enhancement which changes existing behavior).

## Checklist:

- [x] I signed an [Apache CLA](https://github.com/apache/openwhisk/blob/master/CONTRIBUTING.md).
- [x] I reviewed the [style guides](https://github.com/apache/openwhisk/wiki/Contributing:-Git-guidelines#code-readiness) and followed the recommendations (Travis CI will check :).
- [ ] I added tests to cover my changes.
- [ ] My changes require further changes to the documentation.
- [ ] I updated the documentation where necessary.