Crash when mongodb is down

[cdparra]

When I run it with node.js, if by any chance the mongodb server is down, the notification service crashes with any requests that come after. If possible, this should be avoided and some error message should be displayed alerting that the problem is simply that mongodb is down.

macondo:appcivist-notification-service cdparra$ node server.js
mongoUri:  mongodb://localhost:27017/notificationservice
registering event routes with express
registering subscription routes with express
registering signal routes with express
registering log routes with express
About to start listening
Listening on port:  3025
/Users/cdparra/Projects/appcivist/appcivist-notification-service/node_modules/mongodb/lib/mongo_client.js:225
          throw err
          ^

TypeError: Cannot read property 'collection' of null
    at /Users/cdparra/Projects/appcivist/appcivist-notification-service/routes/subscriptions.js:50:24
    at connectCallback (/Users/cdparra/Projects/appcivist/appcivist-notification-service/node_modules/mongodb/lib/mongo_client.js:315:5)
    at /Users/cdparra/Projects/appcivist/appcivist-notification-service/node_modules/mongodb/lib/mongo_client.js:222:11
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)