Show error rate and count at the pipeline level

## What?

The `Pipelines overview` (`/system/pipelines`) page should show the error rate and total for each pipeline, aggregating the values from the rules they contain.

It might also be nice to have this at the pipeline stage level, but that's not quite as important since you'd already be on the pipeline page looking at stages and rules.

## Why?

Having error rates and counts directly on the `Pipelines overview` would make it much easier to identify when pipelines are failing.

`Throughput` is already shown for pipelines listed on the `Pipelines overview` page, and the `Pipeline X` (`/system/pipelines/$id`) page seems to suggest error rates and counts are already being tracked per-rule, per-stage basis.

## Your Environment

* Graylog Version: `6.0.5`