errorHandler.js: error stack first; respond JSON with `url` and `err`
lib/mongodb: actually setup `db`

fix eebb439b50cce37416b8bd22d507175fb65ecc57
config.backend.mongodb: split `url` and `db_name`
lib/mongodb db.on('error'): error stack first (more info)

NOTE: fatal errors and/or crashes inside DB callbacks can not use
      `res.json()` to report UI and that. Timeout will fire in UI
lib/mongodb: fix reconnection callback to pass arguments