Add MongoDB implementation for dropwizard-application-errors?

[sleberknight]

For our services that use Mongo, right now we just use the in-memory H2 implementation. Probably would be useful to have a native Mongo implementation.

[sleberknight]

We're trying to move away from Mongo, not towards it. Therefore, closing this because we're not going to waste time implementing something we have zero use for anymore.