Reset error counter for pipeline rules

It would be helpful to have the option to reset the error counter for a pipeline rule.

## Expected Behavior

Provide an option to reset the counter back to 0.

## Current Behavior

There is no reset function, and the error counter keeps going up. As a work-around, I'm assuming (but haven't tested) I could delete the rule, then recreate it with the same name.

## Possible Solution

Add a "Reset" hyperlink or button beside any (or a few specific) error counters for pipeline rules.

## Context

When troubleshooting a pipeline rule, and I make a change to fix errors generated by the rule, I currently have to make a note of what the error counter was before I saved the changes to the rule. It would be helpful to have the option to reset that error counter after I made a change so I know if there are still errors being generated.

## Your Environment

* Graylog Version: 2.4.6+ceaa7e4
* Elasticsearch Version: 2.4.6
* MongoDB Version: 2.6.10
* Operating System: Ubuntu 16.04.5 LTS
* Browser version: Chrome 70.0.3538.67