Allow to override the unhandledRejection handler

[mgol]

DangerJS specifies an unhandledRejection handler that makes the process finish with an error code.

In our team some of our Danger checks fail for reasons unrelated to the PR reported; for example, some GitHub API endpoint failed or there's a logic error in the checking code. These cases make Danger fail the build and, hence, disallows the PR from being merged.

I'd like to be able to override the behavior so that errors in the checker code are only producing warnings for other people to look at but that wouldn't prevent PRs from being merged. Would adding such a customization be possible?