Publishing silently fails and closes PR when repo has required reviews enabled

[verythorough]

- Do you want to request a feature or report a bug?
bug

- What is the current behavior?
If the publish branch is a protected branch on GitHub, with required reviews enabled, clicking the publish button in the CMS will silently fail, close the PR, and delete the branch, unless the user has the ability to dismiss the review requirement.

The netlify-cms repo happens to be a repo that works like this. Required reviews are enabled, but admin users (not write-permitted users) can override and merge without review. The config for netlifycms.org/admin uses the github backend, so users log in with their own GitHub accounts.

If a write-only collaborator (like @tech4him1, who took the screenshot below) tries to publish a change, GitHub sends the following response:

The CMS ignores the response, assumes success, and deletes the branch and closes the PR.

If a repo admin (like me; I took the next screenshot) tries to publish, everything works as normal:

- What is the expected behavior?
When branch protections are in place and the user (or git-gateway user token) doesn't have permission to override them, the CMS should leave the PR and branch as they are, and display a relevant error message.

- Please link or paste your config.yml below if applicable.
See https://github.com/netlify/netlify-cms/blob/master/website/site/static/admin/config.yml 😉

[tech4him1]

Screenshot showing request methods and response codes: