sharejs fails silently when mongodb database down

when mongodb isn't available sharejs does not throw any messages. Instead just failing silently in weird ways.

It is easy enough to add an error message at [line 37](https://github.com/josephg/ShareJS/blob/master/src/server/db/mongo.coffee#L37)

but that only will catch errors when you are trying to login with a username/password.
Why is .open() only being called when a username and password are required [line 35](https://github.com/josephg/ShareJS/blob/master/src/server/db/mongo.coffee#L35)? Could I add an error message at [line 37](https://github.com/josephg/ShareJS/blob/master/src/server/db/mongo.coffee#L37) and get rid of the test at [line 35](https://github.com/josephg/ShareJS/blob/master/src/server/db/mongo.coffee#L35)?

If that's ok, I'll make a pull request. save someone else a bunch of nasty debugging.