Undefined exception if constructor fails

[nolanlawson]

RiakDOWN was seeing a pouch is not defined here: https://github.com/nolanlawson/pouchdb-all-dbs/blob/master/index.js#L81. It crashed the entire pouchdb-server. That's bad; we should just recover and do something if the constructor fails.

[gustavohenke]

I'm using Cloudant, and they block _all_dbs for unauthenticated users.
So, this plugin causes the same error for me, but on line 119.