[NEW] Check for database existence before starting backup. This provides for
better error messages for existence, authentication, and _bulk_get problems.
[FIXED] Intermittent issues with multiple callbacks, particularly noticeable
when using Node.js 10.
[FIXED] Issue where a success message could confusingly be output after a
fatal error.
[UPGRADED] Increased nodejs-cloudant dependency minimum to 2.2.x.