Probot Pull Request checks should verify Custom Properties exist

## Problem Description
When creating a configuration file for a sub-org or repo you are able to define custom properties that you would like to set on the repo. Currently you are able to define custom property names that do not exist in the org which results in nothing happening when the PR is merged.

### What is actually happening
When a PR is raised the probot checks the changes and shows that that the custom properties will be added to the repo however when the PR is merged the custom properties never get set because they do not exist at the org level.

### What is the expected behavior
The expected behavior would be that the check in the PR fails with an error if either the property name or its value are not valid. The custom property may not exist or in the case of a single or multi-select custom property the value selected is not one of the options that were defined for the custom property.

### Error output, if available

n/a

## Context

### Are you using the hosted instance of probot/settings or running your own?
Running our own using AWS lambda function and API gateway

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

#### Version of probot/settings
2.1.14-rc.1

#### Version of GitHub Enterprise
n/a