No validation of team names

## Problem Description

### What is actually happening
When using a wrong team (or even collaborators) in the `teams` section, the user does not get any information about that in the pull request.

### What is the expected behavior
It would be great if such errors could be discovered during the pull request and added as inline comments on the affected code lines.

### Error output, if available

## Context

### Are you using the hosted instance of probot/settings or running your own?
Self-hosted

### If running your own instance, are you using it with github.com or GitHub Enterprise?
GitHub.com

#### Version of probot/settings
2.0.17

#### Version of GitHub Enterprise