PR Header doesn't handle errors in PR mergeability request

pull-request-can-merge.js doesn't supply a back() handler for PR_CHECK_MERGEABILITY. This means if the REST request fails, we get an error as a rollback is attempted and calls a function that does not exist.

See: https://answers.atlassian.com/questions/39440236/typeerror-e-is-not-a-function

Workaround:

• Determine why the merge check request is failing and fix it.
• This may be a knock on from some other bug. Some reasons why the check might fail:
  • the PR is MERGED or DECLINED. No request should have been made, but perhaps a plugin makes one or our own code has a bug that causes it to make one.
  • The current user has insufficient permissions to the repo, the PR has been removed, or the repo or project was removed.
  • A PullRequestMergeabilityPropertyProvider from a plugin is throwing errors