Handle Mongoose errors gracefully

[vergenzt]

On lines 99, 120, and 127 in passport-local-mongoose.js, the library calls self.save() without any callback function to handle errors in saving. This led to a particularly hard-to-track-down bug for me as I've been working on my PR for #78. :(

(It turns out in the project in which I've been testing it, we had changed the User schema to make a previously optional field required, and it turned out that mongoose's validation was preventing the save, and this library couldn't update the attempts field.)

No biggie, but just thought I'd let you know.

[mvoorberg]

This is a repeat of #58

[nikwen]

Seems to be fixed to me. If I missed something, correct me, otherwise this can be closed.